pkg/homedir: deprecate GetShortcutString() utility
This function was last used in the pkg/mflag package, which was removed
in 14712f9ff0d20a3b64a60103608b8cc998909242, and is no longer used in
libnetwork code since https://github.com/moby/libnetwork/commit/e6de8aec2fc3cea888f620de490fc6ab83700878
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>