gitattributes: drop support for GIT_ATTR_NOGLOBAL

gitattributes: drop support for GIT_ATTR_NOGLOBAL

test-lib sets $HOME to protect against pollution from user settings,
so setting GIT_ATTR_NOGLOBAL would be redundant.  Simplify by
eliminating support for that environment variable altogether.

GIT_ATTR_NOGLOBAL was introduced in v1.7.4-rc0~208^2 (Add global and
system-wide gitattributes, 2010-09-01) as an undocumented feature for
use by the test suite.  It never ended up being used (neither within
git.git nor in other projects).

This patch does not affect GIT_ATTR_NOSYSTEM, which should still be
useful.

Helped-by: Jeff King <redacted>
Signed-off-by: Jonathan Nieder <redacted>
Signed-off-by: Junio C Hamano <redacted>