Code replicated in compression and decompression moved to shared headers

`CHECK_F` macro moved to `error_private.h` (shared between `fse_compress.c` and `fse_decompress.c`). `ZSTD_limitCopy()` moved to `zstd_internal.h` (shared between `zstd_compress.c` and `zstd_decompress.c`). Erroneous build artefact `zstd.h` removed from repo.

lib/compress/fse_compress.c

@@ -625,11 +625,6 @@ size_t FSE_compress_usingCTable (void* dst, size_t dstSize,
 
 size_t FSE_compressBound(size_t size) { return FSE_COMPRESSBOUND(size); }
 
-#define CHECK_V_F(e, f) size_t const e = f; if (ERR_isError(e)) return e
-#ifndef CHECK_F
-#define CHECK_F(f)   { CHECK_V_F(_var_err__, f); }
-#endif
-
 /* FSE_compress_wksp() :
  * Same as FSE_compress2(), but using an externally allocated scratch buffer (`workSpace`).
  * `wkspSize` size must be `(1<<tableLog)`.

lib/compress/zstd_compress.c

@@ -3749,17 +3749,6 @@ static size_t ZSTD_nextInputSizeHint(const ZSTD_CCtx* cctx)
     return hintInSize;
 }
 
-#ifndef ZSTD_DECOMPRESS_INTERNAL_H /* zstd_decompress.c has the same function */
-MEM_STATIC size_t ZSTD_limitCopy(void* dst, size_t dstCapacity, const void* src, size_t srcSize)
-{
-    size_t const length = MIN(dstCapacity, srcSize);
-    if (length > 0) {
-        memcpy(dst, src, length);
-    }
-    return length;
-}
-#endif
-
 /** ZSTD_compressStream_generic():
  *  internal function for all *compressStream*() variants
  *  non-static, because can be called from zstdmt_compress.c