Remove ignored packed attribute and static_assert on Node size

This static_assert is not necessary for the code to work and is
not valid on every platform.

src/regex_impl.cc

@@ -74,18 +74,14 @@ struct ParsedRegex
     };
 
     using NodeIndex = int16_t;
-    struct [[gnu::packed]] Node
+    struct Node
     {
         Op op;
         bool ignore_case;
         NodeIndex children_end;
         Codepoint value;
         Quantifier quantifier;
-        uint16_t filler = 0;
     };
-#ifndef __ppc__
-    static_assert(sizeof(Node) == 16, "");
-#endif
 
     Vector<Node, MemoryDomain::Regex> nodes;