removed ZSTD_decompressBegin() from ZSTD_initDCtx_internal()

It does not feel "right" from a dependency perspective.
ZSTD_initDCtx_internal() is triggered once, on DCtx creation,
while ZSTD_decompressBegin() is invoked at the beginning of each new frame,
and is also a user-facing prototype.

Downside : a DCtx must be init before first usage !
This was always the intention by the way, and is documented as such.
This stage is automatically done within ZSTD_decompress() and variants,
and also within ZSTD_decompressStream().
Only ZSTD_decompressContinue() is impacted,
it must be preceded by a ZSTD_decompressBegin(), as detailed in doc.

A test has been fixed, to no longer rely on undocumented assumption that ZSTD_decompressBegin() is invoked during init.

tests/fullbench.c

@@ -376,6 +376,7 @@ static size_t benchMem(const void* src, size_t srcSize, U32 benchNb)
             skippedSize = frameHeaderSize + ZSTD_blockHeaderSize;
             memcpy(buff2, dstBuff+skippedSize, g_cSize-skippedSize);
             srcSize = srcSize > 128 KB ? 128 KB : srcSize;    /* speed relative to block */
+            ZSTD_decompressBegin(g_zdc);
             break;
         }
     case 32:   /* ZSTD_decodeSeqHeaders */