Remove VARIANT_INLINE

Start in include/mapbox/variant.hpp at the referenced lines and search the repository for VARIANT_INLINE uses. Remove the macro and its references, then verify that the C++11/C++14 library still builds and its existing tests pass.

It appears that some inadvertent changes have disabled the effect of the VARIANT_INLINE macro in debug builds on Windows and release builds on all other platforms, by commenting out the replacement text of the macro.

https://github.com/mapbox/variant/blob/256ddd55582bb7c06c342315dbacc6a42fee4b34/include/mapbox/variant.hpp#L39
https://github.com/mapbox/variant/blob/256ddd55582bb7c06c342315dbacc6a42fee4b34/include/mapbox/variant.hpp#L43

In those cases, the methods tagged with VARIANT_INLINE were inlined or not according to the default behavior of the compiler and optimization settings.

In my tests with mapbox-gl-native, removing the comments and thus restoring VARIANT_INLINE to its intended function in the release build causes a 2.5% size increase in the resulting binary (Apple clang 9, building with -Os):

     VM SIZE                                 FILE SIZE
 --------------                           --------------
  +3.9%  +116Ki __TEXT,__text              +116Ki  +3.9%
  +283% +2.50Ki Table of Non-instructions +2.50Ki  +283%
  +2.2% +1.36Ki Code Signature            +1.36Ki  +2.2%
   +11%    +368 [__LINKEDIT]                    0  [ = ]
  +0.0%     +48 __TEXT,__const                +48  +0.0%
  -1.2%    -224 Function Start Addresses     -224  -1.2%
  -2.2% -1.66Ki __TEXT,__unwind_info      -1.66Ki  -2.2%
 -56.1% -1.89Ki [__TEXT]                  -1.89Ki -56.7%
  -2.3% -9.11Ki __TEXT,__gcc_except_tab   -9.11Ki  -2.3%
  +2.5%  +108Ki TOTAL                      +107Ki  +2.5%

I suggest we remove the VARIANT_INLINE macro entirely, the rationale being:

• The compiler is best positioned to determine whether or not to inline these methods, based on program analysis and the developer's chosen optimization settings.
• VARIANT_INLINE has effectively been a no-op in release builds on the majority of platforms since 372d7c88fe796a138d0e578328914ac80e5a949a, and we haven't noticed any issues with that.
• Restoring it to force inlining in release builds seems to increase the binary size, the opposite of the desired effect.

cc @artemp @springmeyer @lightmare