[Ruby] Fix remove of prefix from method name

The prefix is now only removed at the start.

Fixes https://sourceforge.net/p/swig/bugs/1136/

Examples/test-suite/ruby_naming_bugs.i

@@ -0,0 +1,9 @@
+%module ruby_naming_bugs
+
+%inline %{
+  // Prior to SWIG 4.1.0 the "Cool_" here was overzealously removed while
+  // trying to remove a class name prefix.
+  struct Cool {
+    int somethingCool_Fast() { return 42; }
+  };
+%}