In commit bfee93c7, repair verbs were moved to IDL. During this refactoring,
the `gc_clock.hh` header became unused as its references were relocated.
`clang-include-cleaner` helped identify this unnecessary include, which is
now removed to clean up the codebase.
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closes scylladb/scylladb#21919