bash: remove always true if statement from __git_ps1()

bash: remove always true if statement from __git_ps1()

The recent commits 8763dbb1 (completion: fix PS1 display during a
merge on detached HEAD, 2009-05-16), ff790b6a (completion: simplify
"current branch" in __git_ps1(), 2009-05-10), and d7107ca6
(completion: fix PS1 display during an AM on detached HEAD,
2009-05-26) ensure that the branch name in __git_ps1() is always set
to something sensible.  Therefore, the condition for checking the
non-empty branch name is always fulfilled, and can be removed.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Acked-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>