Remove redefinition of SubscriberImpl

[gartnera]

clangd does not like the redefinition of this symbol. SubscriberImpl is first declared as a class. I don't see any reason why this is actually necessary.

[{
	"resource": "/workspace/.cache/bazel/_bazel_ag/947520f8c2d295d4c795226754f7f58e/external/com_github_agtonomy_trellis/trellis/core/subscriber.hpp",
	"owner": "_generated_diagnostic_collection_name_#0",
	"code": "redefinition_different_kind",
	"severity": 8,
	"message": "Redefinition of 'SubscriberImpl' as different kind of symbol",
	"source": "clang",
	"startLineNumber": 329,
	"startColumn": 1,
	"endLineNumber": 329,
	"endColumn": 6,
	"relatedInformation": [
		{
			"startLineNumber": 42,
			"startColumn": 7,
			"endLineNumber": 42,
			"endColumn": 21,
			"message": "Previous definition is here",
			"resource": "/workspace/.cache/bazel/_bazel_ag/947520f8c2d295d4c795226754f7f58e/external/com_github_agtonomy_trellis/trellis/core/subscriber.hpp"
		}
	],
	"origin": "extHost2"
}]