media: dvb-frontends: lgdt3306a.c: remove dead code

media: dvb-frontends: lgdt3306a.c: remove dead code

Fix the following coccinelle report:

drivers/media/dvb-frontends/lgdt3306a.c:718:5-8:
Unneeded variable: "ret". Return "0" on line 744

Code was enclosed with #if 0 and thus this variable was not needed.
Remove that function altogether.

The  lgdt3306a_spectral_inversion function was dropped completely
since the function doesn't do anything. A comment was left at its
calling site to let others know that spectral_inversion defaults
already set for VSB and QAM

Found using - Coccinelle (http://coccinelle.lip6.fr)

Link: https://lore.kernel.org/linux-media/20200903131311.442255-1-dwlsalmeida@gmail.com
Signed-off-by: Daniel W. S. Almeida <dwlsalmeida@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>