Apply the Protobuf targets cleanups from #4923 to IceRpc.Slice.Tools.targets

[pepone]

Follow-up suggested in the review of #4930: IceRpc.Slice.Tools.targets should take the cleanups made to the Protobuf targets in #4923. The two files mirror each other, and the Protobuf side now has:

• _SlicecPruneStaleOutputs with BeforeTargets="Slicec;SlicecClean" only. A BeforeTargets target runs even when its target's Condition is false, and SlicecClean has no condition, so CoreCompile and Clean in the list are redundant.
• <Compile Remove="@(_StaleSlicecOutput)" MatchOnMetadata="FullPath" MatchOnMetadataOptions="PathLike" /> instead of the _StaleSlicecOutputRelative item, its comment and the MakeRelative call. MatchOnMetadata matches the SDK glob's relative identities directly. The unquoted MakeRelative(..., %(Identity)) also splits on a comma in the path and fails with MSB4186.
• No MakeDir in _SlicecWriteOutputManifest: WriteLinesToFile creates the directory, and PrepareForBuild already created it.
• No Exists guard on the manifest Delete in SlicecClean: Delete skips missing files.
• WriteOnlyWhenDifferent="true" on the manifest WriteLinesToFile.

Behavior to keep, verified on the Protobuf side: renaming a Slice file removes the glob's Compile entry and deletes the old outputs; removing the last Slice file still prunes and deletes the manifest; Rebuild and Clean unchanged.