meson: compile mpv.exe as a win32 gui application

Some compiler flags were passed to mpv in order to get it to build as a
gui application on windows. However on the meson build, this always
resulted in mpv being built as a console application. This is because
the function for making executables in meson specifically has a kwarg
called win_subsystem* which defaults to 'console'. That always added
link arguments at the end which compiled mpv.exe as a console
application. The correct thing to do is to remove all of the
subsystem-related flags in the meson build and use the win_subsystem
kwarg as intended by setting it to 'windows,6.0'. For mpv.com, we can
remove the -Wl,--subsystem,console flag since meson will set this by
default in that executable. This makes mpv.exe function correctly and
open with the pseudo-gui while mpv.com acts as a console wrapper.

https://github.com/mesonbuild/meson/commit/1a0603835e3c9f1047d9b7694efc996219a422e4