Win32: Remove timeGetTime fallback for timer

The performance counter API is guaranteed to succeed on Windows XP and
later so there is no need for a fallback.

This removes our last dependency on winmm.

(cherry picked from commit b6834bf2a1)

src/win32_platform.h

@@ -218,10 +218,6 @@ typedef enum
  #define DIDFT_OPTIONAL 0x80000000
 #endif
 
-// winmm.dll function pointer typedefs
-typedef DWORD (WINAPI * PFN_timeGetTime)(void);
-#define timeGetTime _glfw.win32.winmm.GetTime
-
 // xinput.dll function pointer typedefs
 typedef DWORD (WINAPI * PFN_XInputGetCapabilities)(DWORD,DWORD,XINPUT_CAPABILITIES*);
 typedef DWORD (WINAPI * PFN_XInputGetState)(DWORD,XINPUT_STATE*);
@@ -350,11 +346,6 @@ typedef struct _GLFWlibraryWin32
     int                 rawInputSize;
     UINT                mouseTrailSize;
 
-    struct {
-        HINSTANCE                       instance;
-        PFN_timeGetTime                 GetTime;
-    } winmm;
-
     struct {
         HINSTANCE                       instance;
         PFN_DirectInput8Create          Create;
@@ -422,7 +413,6 @@ typedef struct _GLFWcursorWin32
 //
 typedef struct _GLFWtimerWin32
 {
-    GLFWbool            hasPC;
     uint64_t            frequency;
 } _GLFWtimerWin32;