bash: Remove completion of core.legacyheaders option

This option is no longer recognized by git.  Completing it is
not worthwhile.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

contrib/completion/git-completion.bash

@@ -961,7 +961,6 @@ _git_config ()
 		core.sharedRepository
 		core.warnAmbiguousRefs
 		core.compression
-		core.legacyHeaders
 		core.packedGitWindowSize
 		core.packedGitLimit
 		clean.requireForce