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/API/Syscall.h

@@ -205,7 +205,7 @@ enum Function {
         __Count
 };
 
-inline constexpr const char* to_string(Function function)
+constexpr const char* to_string(Function function)
 {
     switch (function) {
 #undef __ENUMERATE_SYSCALL

Kernel/Arch/i386/CPU.h

@@ -499,7 +499,7 @@ struct [[gnu::aligned(16)]] FPUState
     u8 buffer[512];
 };
 
-inline constexpr FlatPtr page_base_of(FlatPtr address)
+constexpr FlatPtr page_base_of(FlatPtr address)
 {
     return address & PAGE_MASK;
 }
@@ -509,7 +509,7 @@ inline FlatPtr page_base_of(const void* address)
     return page_base_of((FlatPtr)address);
 }
 
-inline constexpr FlatPtr offset_in_page(FlatPtr address)
+constexpr FlatPtr offset_in_page(FlatPtr address)
 {
     return address & (~PAGE_MASK);
 }

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);
 }