Remove unused slider component

It is unused as of matrix-org#12246. I noticed this while working on matrix-org#12299.

Author: robintown

res/css/_components.pcss

@@ -209,7 +209,6 @@
 @import "./views/elements/_SearchWarning.pcss";
 @import "./views/elements/_ServerPicker.pcss";
 @import "./views/elements/_SettingsFlag.pcss";
-@import "./views/elements/_Slider.pcss";
 @import "./views/elements/_Spinner.pcss";
 @import "./views/elements/_StyledCheckbox.pcss";
 @import "./views/elements/_StyledRadioButton.pcss";

res/css/views/elements/_Slider.pcss

@@ -251,8 +251,6 @@ $progressbar-bg-color: $system;
 $kbd-border-color: $strong-input-border-color;
 $visual-bell-bg-color: #800;
 $event-timestamp-color: $text-secondary-color;
-$slider-background-color: $quinary-content;
-$appearance-tab-border-color: $room-highlight-color;
 $composer-shadow-color: rgba(0, 0, 0, 0.28);
 $breadcrumb-placeholder-bg-color: #272c35;
 $theme-button-bg-color: #e3e8f0;

res/themes/dark/css/_dark.pcss

@@ -208,9 +208,6 @@ $breadcrumb-placeholder-bg-color: #272c35;
 $voice-record-stop-border-color: #6f7882;
 $voice-record-icon-color: #6f7882;
 
-/* Appearance tab colors */
-$appearance-tab-border-color: $room-highlight-color;
-
 $composer-shadow-color: tranparent;
 
 $codeblock-background-color: #2a3039;

res/themes/legacy-dark/css/_legacy-dark.pcss

@@ -269,9 +269,6 @@ $visual-bell-bg-color: #faa;
 $togglesw-off-color: #c1c9d6;
 $togglesw-ball-color: var(--cpd-color-bg-action-primary-rest);
 
-/* Slider */
-$slider-background-color: #c1c9d6;
-
 $progressbar-bg-color: rgba(141, 151, 165, 0.2);
 
 $authpage-bg-color: #2e3649;
@@ -305,9 +302,6 @@ $voice-record-live-circle-color: #ff4b55;
 $voice-record-stop-border-color: #e3e8f0;
 $voice-record-icon-color: $tertiary-fg-color;
 
-/* FontSlider colors */
-$appearance-tab-border-color: $input-darker-bg-color;
-
 $composer-shadow-color: tranparent;
 
 $codeblock-background-color: $header-panel-bg-color;

res/themes/legacy-light/css/_legacy-light.pcss

@@ -31,7 +31,6 @@ $button-secondary-bg-color: $accent-fg-color;
 $message-action-bar-fg-color: $primary-content;
 $voice-record-stop-border-color: $quinary-content;
 $voice-record-icon-color: $tertiary-content;
-$appearance-tab-border-color: $input-darker-bg-color;
 $eventbubble-reply-color: $quaternary-content;
 $roomtopic-color: $secondary-content;
 
@@ -104,10 +103,6 @@ $accent-1400: var(--cpd-color-green-1400);
     background-color: $panel-actions !important;
 }
 
-.mx_FontScalingPanel_fontSlider {
-    background-color: $panel-actions !important;
-}
-
 .mx_ThemeChoicePanel_themeSelectors > .mx_StyledRadioButton input[type="radio"]:disabled + div {
     border-color: $primary-content;
 }

res/themes/light-high-contrast/css/_light-high-contrast.pcss

@@ -316,8 +316,6 @@ $progressbar-bg-color: $panel-actions;
 $kbd-border-color: $strong-input-border-color;
 $visual-bell-bg-color: #faa;
 $event-timestamp-color: #acacac;
-$slider-background-color: $togglesw-off-color;
-$appearance-tab-border-color: $input-darker-bg-color;
 $composer-shadow-color: rgba(0, 0, 0, 0.04);
 $breadcrumb-placeholder-bg-color: #e8eef5;
 $theme-button-bg-color: $quinary-content;