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.

Kernel/FileSystem/InodeMetadata.h

@@ -35,7 +35,7 @@ namespace Kernel {
 
 class Process;
 
-inline constexpr u32 encoded_device(unsigned major, unsigned minor)
+constexpr u32 encoded_device(unsigned major, unsigned minor)
 {
     return (minor & 0xff) | (major << 8) | ((minor & ~0xff) << 12);
 }