completion: Drop init_completion define

Changeset cd8084f9 ("locking/lockdep: Apply crossrelease to completions")
added a CONFIG_LOCKDEP_COMPLETE (that was later renamed to
CONFIG_LOCKDEP_COMPLETIONS).

Such changeset renamed the init_completion, and add a macro
that would either run a modified version or the original code.

However, such code reported too many false positives. So, it
ended being dropped later on by
changeset e966eaee ("locking/lockdep: Remove the cross-release locking checks").

Yet, the define remained there as just:

	 #define init_completion(x) __init_completion(x)

Get rid of the define, and return __init_completion() function
to its original name.

Fixes: e966eaee ("locking/lockdep: Remove the cross-release locking checks")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/e657bfc533545c185b1c3c55926a449ead56a88b.1606823973.git.mchehab+huawei@kernel.org