Remove xrdp_sec_in_mcs_data() function

[matt335672]

Back in #1742, @aquesnel added these comments to the header of xrdp_sec_in_mcs_data():-

/* TODO: why does this method parse the strust from back to front (resetting after each field) instead of from front to back like the rest of the parsing code? /
/ TODO: why does this method exist when the same struct is parsed in xrdp_sec_process_mcs_data_CS_CORE and there does not seem to be any dependencies preventing a call to that function. /
/ TODO: this is a brittle function that assumes field offsets in the stream instead of parsing the variable length fields of [ITU T.124] ConferenceCreateRequest */

I've finally gotten round to looking into this, and, like @aquesnel, I've reached the conclusion this function is not necessary (and is, in fact, confusing). The function parses data within the TS_UD_CS_CORE struct which could just as easily be parsed when xrdp_sec_process_mcs_data_CS_CORE() is called earlier.

This ASCII art show the call sequence of these functions:-

xrdp_sec_incoming()
    |
    +-xrdp_mcs_incoming()
    |   |
    |   +-xrdp_sec_process_mcs_data()
    |       |
    |       +-xrdp_sec_process_mcs_data_CS_CORE()
    . . .
    |
    +-xrdp_sec_in_mcs_data()

Currently the contents of the MSC Connect Initial PDU are stored within the client_mcs_data member of the xrdp_sec struct. This stream is parsed once by xrdp_sec_process_mcs_data() and then separately by xrdp_sec_in_mcs_data() (as the diagram above shows). There is no reason not to perform the parse in a single pass through the stream.

This commit folds the functionality in xrdp_sec_in_mcs_data() into xrdp_sec_process_mcs_data_CS_CORE() and removes xrdp_sec_in_mcs_data()

Looking back on the history of these functions, xrdp_sec_in_mcs_data() is the older, and was added in August 2005 by 5e69f15. xrdp_sec_process_mcs_data_CS_CORE() was originally called xrdp_rdp_parse_client_mcs_data_CS_CORE() and was added in Feb 2014 by 1d9c773 to a different file. The two functions have never been combined.