'pastetoggle': Revert support for multi-key value (#6724)

Reverts commit 337b6179df

Closes #6716 at the expense of not being able to use a
multi-key 'pastetoggle' manually.

Multi-key 'pastetoggle' can still be used when inserting the entire
option into the typebuffer at once (though the use here is
questionable).

Also remove those tests to do with waiting for the completion of
'pastetoggle' and mention in the documentation that 'pastetoggle'
doesn't wait for timeout.

src/nvim/getchar.c

@@ -1908,7 +1908,7 @@ static int vgetorpeek(int advance)
           }
 
           if ((mp == NULL || max_mlen >= mp_match_len)
-              && keylen != KEYLEN_PART_MAP && keylen != KEYLEN_PART_KEY) {
+              && keylen != KEYLEN_PART_MAP) {
             // No matching mapping found or found a non-matching mapping that
             // matches at least what the matching mapping matched
             keylen = 0;