Remove unnecessary parser complexity

This `!"="` is necessary for general macro calls, to prevent
accidentally considering `foo != bar` as if it were a macro call going
`foo! =` and then getting confused. However, both the `calc` and `seq`
macros explicitly check for the curly braces, so don't actually need the
negative forward lookahead.

src/verus.pest

@@ -498,12 +498,12 @@ calc_macro_body = {
 }
 
 calc_macro_call = {
-    attr* ~ calc_str ~ bang_str ~ !"=" ~
+    attr* ~ calc_str ~ bang_str ~
     calc_macro_body
 }
 
 seq_macro_call = {
-    attr* ~ seq_str ~ bang_str ~ !"=" ~
+    attr* ~ seq_str ~ bang_str ~
     lbracket_str ~ comma_delimited_exprs? ~ rbracket_str
 }