Remove some ununsed/unnecessary compile flags and some dead code

There has been an accumulation of compile FLAGS in the code through years of development. The following issue is dedicated to identification and removal of unused/unnecessary compile flags. Details for each compile FLAG being removed is detailed below. The list will evolve until the issue is resolved. Comments are welcome. Note that results of FLAG removal might lead to:

-code change

-build change

-file removal

Flag USER_MODE:

Identifies code that will run in user space. All code (except driver code) runs in user space, the flag is not necessary. Driver code is not flagged and built specifically.

Flags RTAI, RTAI_ENABLED, ENABLE_RTAI_CLOCK, RTAI_ISNT_POSIX, _RTAI_MATH_H:

A set of related flags which would be needed to compile OAI to make it run on RTAI-patched Linux kernel. The usage of RTAI-patched Linux kernel is discontinued in OAI. Only vanilla Linux kernels (generic and lowlatency) are supported by OAI.

Flag CELLULAR, TIMER_ONESHOT_MODE:

Looks to be useless in the header files. Got confirmed it can be removed. {OPENAIR2_DIR}/NAS can be entirely removed and any reference to it (i.e. header file can be removed).

Flag EMOS, EMOS_CHANNEL:

Useless. Remove any usage and files/directories related to EMOS.

Flag OAI_EMU, OPENAIR_EMU, PHY_ABSTRACTION, PHY_ABSTRACTION_UL:

Useless. Remove any usage and files/directories related to OAI_EMU.

RRH gateway dead code and related files

Flag DISABLE_USE_NAS:

Useless. Remove any reference in code and build.

Flag ENABLE_FXP: