Remove some unreachable code lines from _is_table_equals()

Coverage analysis indicates that the test for an "empty"
actualKeysMatched table always succeeds, and the enclosed
return statement never gets executed.

This is to be expected, as for any given (non table-type)
key `k` that is present in `actual` but not in `expected`,
our earlier value-based test would already fail. We compare
actual[k] against (expected[k] == nil), which won't pass.

Fixed this by removing the unnecessary code.

luaunit.lua

@@ -698,18 +698,10 @@ local function _is_table_equals(actual, expected, recursions)
                     -- Found a key that we did not see in "actual" -> mismatch
                     return false
                 end
-                -- Otherwise we know that actual[k] was already matched
-                -- against v = expected[k]. Remove k from the table again.
-                actualKeysMatched[k] = nil
+                -- Otherwise actual[k] was already matched against v = expected[k].
             end
         end
 
-        -- If we have any keys left in the actualKeysMatched table, then those
-        -- were missing from "expected", meaning the tables are different.
-        if next(actualKeysMatched) then
-            return false
-        end
-
         if next(actualTableKeys) then
             -- If there is any key left in actualTableKeys, then that is
             -- a table-type key in actual with no matching counterpart