fmt: fix backslash doubling in attribute string args (#9175)

vlib/v/fmt/tests/attrs_keep.vv

@@ -1,3 +1,8 @@
+struct AttrsWithEscpaedStringArgs {
+	dollar    string [foo: '\$var']
+	double_bs string [bar: '\\baz']
+}
+
 [deprecated: 'use bar() instead']
 [foo: bar]
 [if debug; inline]

vlib/v/table/attr.v

@@ -29,9 +29,7 @@ pub fn (attr Attr) str() string {
 		if attr.arg.len > 0 {
 			s += ': '
 			if attr.is_string_arg {
-				mut a := attr.arg.replace('\\', '\\\\')
-				// FIXME: other escapes e.g. \r\n
-				a = a.replace("'", "\\'")
+				a := attr.arg.replace("'", "\\'")
 				s += "'$a'"
 			} else {
 				s += attr.arg