Fix invalid reset of URL variable in csd-wrapper
The URL variable is constructed from the CSD_HOSTNAME at the beginning of
the script. However, prior to parsing the command line, it was reset to
an empty value.
[DRL: This bug has existed since
https://gitlab.com/openconnect/openconnect/-/commit/
cb83e535213ff2132643d2a68c50abc294b43b82,
when I modified the `csd-wrapper.sh` script to parse its `-url` command-line
argument, but forgot to remove the subsequent line `URL=`.]
Signed-off-by: Audric Schiltknecht <storm+gitlab@chemicalstorm.org>