prep_rtc.validate_slc check is unproductive

[asjohnston-asf]

prep_rtc.validate_slc attempts two checks:

1. That the SLC is single-pol. This check is both invalid (the majority of SLC RTC processing is for dual-pol granules) and incorrectly implemented (checks for the string VH or HV in the granule name, where dual-pol granules would have DV or DH).
2. That the SLC exists in CMR. This check is duplicative and introduces an undesired dependence on CMR for SLC processing. If the granule isn't part of the ASF archive we'll immediately get an HTTP 404 error when attempting to download the granule.