remove synopsis editing hack for RHSAs

[ktdreyer]

In erratum.py we have the following workaround:

# ET automagically handles the severity for the synopsis in RHSA's
# but will still see it as a docs change if we write the same one
# back again, so remove it.
if self.errata_type == 'RHSA':
    severity = r'^(Low|Moderate|Important|Critical): '
    self.synopsis = re.sub(severity, "", self.synopsis)

This ET server behavior might be changing in RHELWF-181. Need to confirm with the latest ET version that this is still an issue. If it's truly resolved, we can drop this workaround code.