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>
diff --git a/contrib/completion/git-completion.bash b/contrib/completion/git-completion.bash
index 2c4bf4e..73ed095 100755
--- a/contrib/completion/git-completion.bash
+++ b/contrib/completion/git-completion.bash
@@ -961,7 +961,6 @@
 		core.sharedRepository
 		core.warnAmbiguousRefs
 		core.compression
-		core.legacyHeaders
 		core.packedGitWindowSize
 		core.packedGitLimit
 		clean.requireForce