Dropped LFS3_KVONLY and LFS3_2BONLY modes for now

I think these are good ideas to bring back when littlefs3 is more
mature, but at the moment the number of different builds is creating too
much friction.

LFS3_KVONLY and LFS3_2BONLY in particular _add_ significant chunks of
code (lfs3_file_readget_, lfs3_file_flushset_, and various extra logic
sprinkled throughout the codebase), and the current state of testing
means I have no idea if any of it still works.

These are also low-risk for introducing any disk related changes.

So, ripping out for now to keep the current experimental development
tractable. May reintroduce in the future (probably after littlefs3 is
stabilized) if there is sufficient user interest. But doing so will
probably also need to come with actual testing in CI.

lfs3_util.h

@@ -56,12 +56,6 @@
 #ifdef LFS3_YES_RDONLY
 #define LFS3_RDONLY
 #endif
-#ifdef LFS3_YES_KVONLY
-#define LFS3_KVONLY
-#endif
-#ifdef LFS3_YES_2BONLY
-#define LFS3_2BONLY
-#endif
 #ifdef LFS3_YES_REVDBG
 #define LFS3_REVDBG
 #endif
@@ -209,18 +203,6 @@
 #define LFS3_IFDEF_RDONLY(a, b) (b)
 #endif
 
-#ifdef LFS3_KVONLY
-#define LFS3_IFDEF_KVONLY(a, b) (a)
-#else
-#define LFS3_IFDEF_KVONLY(a, b) (b)
-#endif
-
-#ifdef LFS3_2BONLY
-#define LFS3_IFDEF_2BONLY(a, b) (a)
-#else
-#define LFS3_IFDEF_2BONLY(a, b) (b)
-#endif
-
 #ifdef LFS3_REVDBG
 #define LFS3_IFDEF_REVDBG(a, b) (a)
 #else