Remove 'Id' field from JSON plugin metadata

KPluginMetaData (since kcoreaddons commit be10ddbf) now derives the
plugin ID from the file name and ignores the ID field.  Eliminates many
runtime warnings of the form:

konqueror/kf.coreaddons KPluginMetaData::KPluginMetaData: The plugin
"/usr/lib64/plugins/kf6/parts/dolphinpart.so" explicitly states an 'Id'
in the embedded metadata. This value should be removed, the resulting
pluginId will not be affected by it

Author: martenjj

src/dolphinpart.json.in

@@ -1,7 +1,6 @@
 {
     "KPlugin": {
         "Icon": "view-list-icons",
-        "Id": "dolphinpart",
         "InitialPreference": 7,
         "License": "LGPL v2+",
         "MimeTypes": [