drm/amd/powerplay: remove set but not used variable 'stretch_amount2'

drivers/gpu/drm/amd/amdgpu/../powerplay/smumgr/vegam_smumgr.c:
 In function vegam_populate_clock_stretcher_data_table:
drivers/gpu/drm/amd/amdgpu/../powerplay/smumgr/vegam_smumgr.c:1489:29:
 warning: variable stretch_amount2 set but not used [-Wunused-but-set-variable]

It is never used, so can be removed.

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