Remove the exporters/zipkin/internal package

[MrAlias]

The only use of the packages contained within the exporters/zipkin/internal package are:

1. opentelemetry-go/exporters/zipkin/zipkin_test.go

   Lines 79 to 85 in b213e25

   	envStore, err := ottest.SetEnvVariables(map[string]string{
   	envEndpoint: expectedEndpoint,
   	})
   	assert.NoError(t, err)
   	defer func() {
   	require.NoError(t, envStore.Restore())
   	}()

2. opentelemetry-go/exporters/zipkin/env_test.go

   Lines 37 to 41 in b213e25

   	envStore := ottest.NewEnvStore()
   	envStore.Record(envEndpoint)
   	defer func() {
   	require.NoError(t, envStore.Restore())
   	}()

• Replace the use in (1) with t.Setenv (e.g. t.Setenv(envEndpoint, expectedEndpoint))
• Remove the reference in (2) (it has not been needed since Add support for go 1.23 #5720)
• Remove the exporters/zipkin/internal package

[tongoss]

@MrAlias , can I take this one?