mingw: remove unneeded `NO_GETTEXT` directive

mingw: remove unneeded `NO_GETTEXT` directive

In f9206ce2681 (mingw: let's use gettext with MSYS2, 2016-01-26), we
flipped the switch to build Git for Windows with support for gettext.

However, the way we flipped the switch was by changing the value of the
`NO_GETTEXT` variable from a non-empty string to the empty string, as if
there was any `NO_GETTEXT` definition we needed to override.

But that was a mistake: while there _is_ a definition, it is in the
`THIS_IS_MSYSGIT` section, i.e. it does not affect the Git for Windows
part at all.

Let's just remove that unnecessary line.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>