[settings] Remove "uristring" setting type

[settings] Remove "uristring" setting type

Commit b5f5f73 ("[cmdline] Expand settings within each command-line
token individually") effectively rendered the "uristring" setting type
obsolete, since strings containing whitespace no longer break the
command line parser.  The concept of the "uristring" type is not well
defined, since URI escaping rules depend on which portion of a URI is
being escaped.

Remove the "uristring" type, converting it into an alias for the
"string" setting type so as to avoid breaking existing scripts.

Signed-off-by: Michael Brown <mcb30@ipxe.org>