git-curl-compat: remove check for curl 7.56.0

git-curl-compat: remove check for curl 7.56.0

libcurl 7.56.0 was released in September 2017, which is over seven years
ago, and no major operating system vendor is still providing security
support for it.  Debian 10, which is out of mainstream security support,
has supported a newer version, and Ubuntu 20.04 and RHEL 8, which are
still in support, also have 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>