Remove superfluous `gnutls-available-p' check

* lisp/url/url-http.el (url-http-parse-headers): `gnutls-available-p' is
now always available.
Ignore directory symlinks in directory-files-recursively

* files.el (directory-files-recursively): Don't follow symlinks to
other directories.
Remove more (fboundp 'gnutls-available-p)

* lisp/net/network-stream.el (network-stream-open-starttls): No need to
check for the availability of `gnutls-available-p'.