mingw: remove unneeded `NO_CURL` directive

mingw: remove unneeded `NO_CURL` directive

In df5218b4c30 (config.mak.uname: support MSys2, 2016-01-13), we
introduced support for building Git for Windows in the then-brand new
Git for Windows v2.x build environment that was based off of MSYS2.

To do that, we split the non-msysGit part (that targeted MSys1) in two,
and instead of sharing the `NO_CURL = YesPlease` setting with MSys1, we
overrode it for MSYS2 with the empty value because we very much want to
build Git for Windows with libcurl.

But that was unnecessary: we never set that variable beforehand,
therefore there is no need to override it.

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>