AK: Remove clear() from FixedArray and fixate its allocation guarantees

FixedArray always *almost* had the following allocation guarantees:
There is (possibly) one allocation in the constructor and one (or more)
deallocation(s) in the destructor. No other operation allocates or
deallocates. With this removal of the public clear() method, which
nobody except the test used anyways, those guarantees are now completely
true and furthermore fixated with an explanatory comment.

Tests/AK/TestFixedArray.cpp

@@ -23,7 +23,4 @@ TEST_CASE(ints)
     EXPECT_EQ(ints[0], 0);
     EXPECT_EQ(ints[1], 1);
     EXPECT_EQ(ints[2], 2);
-
-    ints.clear();
-    EXPECT_EQ(ints.size(), 0u);
 }