cogl-winsys-egl-kms: Never set EGL_PLATFORM
cogl-winsys-egl-kms: Never set EGL_PLATFORM

This environment variable predates the reliable platform detection in mesa
and typically just causes crashes when the specified platform doesn't
match what's passed in.  Aside from being unecessary and problematic
it also leaks into the GNOME session, preventing clients from
automatically detecting the wayland platform.

https://bugzilla.gnome.org/show_bug.cgi?id=728978

Reviewed-by: Neil Roberts <neil@linux.intel.com>

(cherry picked from commit 8fb9cea4515161effdaee256dd033c36cfa21689)

Note while forward-porting: On master this patch only affects the
Wayland winsys. The KMS winsys never set EGL_PLATFORM on master
because that was introduced in commit dab054200cb2135da2303 which was
only applied to the 1.18 branch. That patch was only made to override
the environment variable if the Wayland winsys is tried before the KMS
winsys.

djdeath/cogl,djdeath/cogl,djdeath/cogl