Remove deprecated `decodeBuffered` and `decodeBufferedAsync` functions.

[05nelsonm]

See #224

MUST happen before cutting the 2.6.0 release.

[05nelsonm]

Adding the blocked label until #224 is completed

[05nelsonm]

Also update :core module README.md with the decodeBuffered and decodeBufferedAsync API changes. Forgot to do that in #226

Will be handled in #229