Everywhere: Redundant inline specifier on constexpr functions (#3807)

Problem:
- `constexpr` functions are decorated with the `inline` specifier
  keyword. This is redundant because `constexpr` functions are
  implicitly `inline`.
- [dcl.constexpr], §7.1.5/2 in the C++11 standard): "constexpr
  functions and constexpr constructors are implicitly inline (7.1.2)".

Solution:
- Remove the redundant `inline` keyword.

AK/StringImpl.h

@@ -116,7 +116,7 @@ private:
     char m_inline_buffer[0];
 };
 
-inline constexpr u32 string_hash(const char* characters, size_t length)
+constexpr u32 string_hash(const char* characters, size_t length)
 {
     u32 hash = 0;
     for (size_t i = 0; i < length; ++i) {