Fix for issue 3385135 "source code cleanup" engine part

- Removed below unused functions
    a. M4PTO3GPP_RegisterExternalVideoEncoder
    b. M4VSS3GPP_editRegisterExternalVideoDecoder
    c. M4VSS3GPP_editRegisterExternalVideoEncoder
    d. M4VSS3GPP_extractPictureRegisterExternalVideoDecoder
    e. M4VSS3GPP_editRegisterExternalCodec
    f. M4VSS3GPP_editSubscribeExternalCodecs
    g. M4VSS3GPP_intSubscribeExternalCodecs
    h. M4VSS3GPP_intClipRegisterExternalVideoDecoder
    i. M4xVSS_RegisterExternalVideoDecoder
    j. M4xVSS_RegisterExternalVideoEncoder
    k. M4MCS_registerExternalVideoDecoder
    l. M4MCS_registerExternalVideoEncoder
    m. M4MCS_registerExternalAudioDecoder
    n. M4MCS_registerExternalAudioEncoder
    o. M4DECODER_EXTERNAL_getInterface

- Also remove small code associated with these functions and Remove below unsused src + header files
    a. M4CLOCK.h
    b. M4MDP_API.h
    c. M4VD_EXTERNAL_Internal.h
    d. M4VD_HW_API.h
    e. M4VE_API.h
    f. M4_Logo.h
    g. M4VD_EXTERNAL_Interface.c
    h. LV_Types.h
    i. M4OSA_FileAccess.h
    j. M4OSA_FileCache.h
    k. M4OSA_FileExtra.h
    l. M4OSA_FileReaderRam.h
    m. M4OSA_FileWriterRam.h
    n. M4OSA_String.h
    o. M4OSA_String_priv.h
    p. M4OSA_FileCache.c
    q. M4OSA_FileExtra.c
    r. M4OSA_FileReader_RAM.c
    s. M4OSA_FileWriter_RAM.c
    t. M4OSA_String.c
    u. M4OSA_String_priv.c

Change-Id: I7271cbe92168e665a8e5b7afe7c7fe1c331eaa1e