Makefile: drop warning suppression on win build

Makefile: drop warning suppression on win build

Commit dcdf71c21b99 ("- fix signed/unsigned build errors under
win32. Thanks Zach Welch <zw@superlucidity.net>") in 2009 prevents
gcc warnings on sign/unsigned comparisons while building for Win
on folders 'helper' and 'server'.
In 2011, commit b69119668ed8 ("RTOS Thread awareness support wip")
uses the same method on the new folder 'rtos'.

In mean time, all the incorrect sign/unsigned comparisons has been
fixed and no warning is present with the default -Wextra flag that
implies -Wsign-compare.
The comment:
	# FD_* macros are sloppy with their signs on MinGW32 platform
seems linked to some old implementation of MinGW32 include file
that doesn't apply on current versions.

Remove the obsolete hacks to suppress the warnings.

Change-Id: I76dba9e54a647d3b9fbf1b7e9ae1844e3d7adc9a
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6254
Tested-by: jenkins
Reviewed-by: Xiaofan Chen <xiaofanc@gmail.com>