gccrs: Fix semicolon after some namespace

gccrs: Fix semicolon after some namespace

Remove namespace comment after classes and structs.

gcc/rust/ChangeLog:

	* checks/errors/borrowck/rust-bir-fact-collector.h: Remove spurious
	comment.
	* checks/errors/rust-feature.cc: Likewise.
	* util/optional.h: Likewise.
	* expand/rust-token-tree-desugar.cc (TokenTreeDesugar::visit): Remove
	semicolons on namespace.
	* expand/rust-token-tree-desugar.h: Likewise.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>