drm/amd/display: remove set but not used variable 'mc_vm_apt_default'

drivers/gpu/drm/amd/amdgpu/../display/dc/dcn21/dcn21_hubp.c:
 In function hubp21_set_vm_system_aperture_settings:
drivers/gpu/drm/amd/amdgpu/../display/dc/dcn21/dcn21_hubp.c:343:23:
 warning: variable mc_vm_apt_default set but not used [-Wunused-but-set-variable]

It is never used, so remove it.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>