refactor(mt6795): remove mediatek's mt6795 platform

Mediatek's mt6795 platform was deprecated in 2.5 release and as per [1]
a platform which has been marked deprecated should be removed from repo
after 2 release cycle.

[1] https://trustedfirmware-a.readthedocs.io/en/latest/plat/deprecated.html?highlight=deprecated



Signed-off-by: Manish Pandey <manish.pandey2@arm.com>
Change-Id: Ic427a3071316a13f34a726a1eb086b679e1671a1