Remove test of the unsupported core.legacyHeaders variable

Remove test of the unsupported core.legacyHeaders variable

Long ago we stopped supporting the core.legacyHeaders variable,
as JGit (like C Git) stopped creating the new pack-style loose
objects, rendering this variable pointless.  The test is still
valid, it proves we write the standard loose object format for
a commit, but the variable assignment has no impact on the test
so drop it from the code.

Change-Id: I051336ada23033c05e86bbff73ae5d78a37b1640
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>