media: dvb-frontends/stv0910: remove unneeded symbol rate inquiry

tracking_optimization() doesn't make use of the symbol rate reported by
the demodulator, so remove the unneeded inquiry and the now unneeded
variable.

Reported-by: Richard Scobie <rascobie@slingshot.co.nz>
Cc: Ralph Metzler <rjkm@metzlerbros.de>
Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Tested-by: Richard Scobie <rascobie@slingshot.co.nz>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>