git-curl-compat: remove check for curl 7.21.5

git-curl-compat: remove check for curl 7.21.5

libcurl 7.21.5 was released in April 2011, which is well over ten years
ago, and no major operating system vendor is still providing security
support for it.  Debian 7, RHEL 7, and Ubuntu 12.04, all of which are
out of mainstream security support, have all supported 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>