AK: Remove String-from-StringView optimization

We had an unusual optimization in AK::StringView where constructing
a StringView from a String would cause it to remember the internal
StringImpl pointer of the String.

This was used to make constructing a String from a StringView fast
and copy-free.

I tried removing this optimization and indeed we started seeing a
ton of allocation traffic. However, all of it was due to a silly
pattern where functions would take a StringView and then go on
to create a String from it.

I've gone through most of the code and updated those functions to
simply take a String directly instead, which now makes this
optimization unnecessary, and indeed a source of bloat instead.

So, let's get rid of it and make StringView a little smaller. :^)

AK/StringUtils.cpp

@@ -219,8 +219,6 @@ static inline char to_lowercase(char c)
 
 bool equals_ignoring_case(const StringView& a, const StringView& b)
 {
-    if (a.impl() && a.impl() == b.impl())
-        return true;
     if (a.length() != b.length())
         return false;
     for (size_t i = 0; i < a.length(); ++i) {