Remove support for legacy `$INCLUDE` syntax

Although test files once expressed dependencies on external files using
a global `$INCLUDE` function, that pattern was removed in favor of
declarative meta-data [1].

Remove the associated logic from the Python runner and the browser.

[1] See commit d4354d14d5.

harness/sth.js

@@ -173,12 +173,7 @@ function BrowserRunner() {
             include;
         
         includes = test.includes;
-        if (!includes || !(includes.length)) {
-            // includes not specified via frontmatter;  find all of the $INCLUDE statements
-            includes = code.match(/\$INCLUDE\(([^\)]+)\)/g);
-        }
-
-        if (includes !== null) {
+        if (includes && includes.length) {
             // We have some includes, so loop through each include and
             // pull in the dependencies.
             for (var i = 0; i < includes.length; i++) {

tools/packaging/test262.py

@@ -277,7 +277,7 @@ class TestCase(object):
   def GetIncludeList(self):
     if self.testRecord.get('includes'):
       return self.testRecord['includes']
-    return re.findall('\$INCLUDE\([\'"]([^\)]+)[\'"]\)', self.test)
+    return []
 
   def GetAdditionalIncludes(self):
     return '\n'.join([self.suite.GetInclude(include) for include in self.GetIncludeList()])