git-curl-compat: remove check for curl 7.39.0

git-curl-compat: remove check for curl 7.39.0

libcurl 7.39.0 was released in November 2014, which is almost ten years
ago, and no major operating system vendor is still providing security
support for it.  Debian 9 and Ubuntu 16.04, both of which are out of
mainstream security support, have supported a newer version, and RHEL 8,
which is still in support, also has a newer version.

Remove the check for this version and use this functionality
unconditionally.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>