parser: remove the 'non-string keys are work in progress' warning

vlib/v/checker/tests/map_ops.out

@@ -1,9 +1,3 @@
-vlib/v/checker/tests/map_ops.vv:2:18: warning: non-string keys are work in progress
-    1 | fn test_map() {
-    2 |     mut m := map[int]string
-      |                     ^
-    3 |     _ = m[`!`]
-    4 |     m['hi'] = 8
 vlib/v/checker/tests/map_ops.vv:3:7: error: invalid key: expected `int`, not `rune`
     1 | fn test_map() {
     2 |     mut m := map[int]string

vlib/v/parser/parse_type.v

@@ -85,9 +85,6 @@ pub fn (mut p Parser) parse_map_type() table.Type {
 			p.tok.position())
 		return 0
 	}
-	if key_type != table.string_type_idx {
-		p.warn_with_pos('non-string keys are work in progress', p.tok.position())
-	}
 	p.check(.rsbr)
 	value_type := p.parse_type()
 	if value_type.idx() == 0 {