SUNRPC: Remove PROC_FS dependency

Recently, commit 440bcc59 added a reverse dependency to fs/Kconfig to
ensure that PROC_FS was enabled if SUNRPC_GSS was enabled.

Apparently this isn't necessary because the auth_gss components under
net/sunrpc will build correctly even if PROC_FS is disabled, though
RPCSEC_GSS will not work without /proc.

It also violates the guideline in Documentation/kbuild/kconfig-language.txt
that states "In general use select only for non-visible symbols (no prompts
anywhere) and for symbols with no dependencies."

To address these issues, remove the dependency.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>