remove 2nd exch call in exch2_s3d_rx (since EXCH_IGNORE_CORNERS is now  implemented in scalar exchange (rx1)).
remove 2nd exch call in exch2_s3d_rx (since EXCH_IGNORE_CORNERS is now
 implemented in scalar exchange (rx1)).

altMITgcm/MITgcm66h,altMITgcm/MITgcm66h,altMITgcm/MITgcm66h,altMITgcm/MITgcm66h,altMITgcm/MITgcm66h,altMITgcm/MITgcm66h,altMITgcm/MITgcm66h,altMITgcm/MITgcm66h