drm/amd/display: Clean FPGA code in dc

[Why]
Drop dead code for Linux.

[How]
Remove all IS_FPGA_MAXIMUS_DC and IS_DIAG_DC

Reviewed-by: Ariel Bernstein <eric.bernstein@amd.com>
Acked-by: Tom Chung <chiahsuan.chung@amd.com>
Signed-off-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>