update antenna.rst file with Sardara configuration

Sardara spectro-polarimetric mode is available with L-Band, so I deleted all the notes where it was written that it isn't available. Also, I removed the backend IF  filters in L-Band (112,460,250) which weren't available and they won't be in future. Made more cleare the Sardara backend table. 
 Removed 3c48 as calibrator which is flaring.