memtx: drop UNCHANGED (get = get_raw) index vtab optimization

We use a special, less efficient index vtab if a space can store
compressed tuples. The problem is it isn't enough to look at a space
definition to figure out if there are compressed tuples in the space:
there may be compressed tuples left from before the alter operation that
disabled compression, since we don't rebuild tuples on alter. To update
an index vtab dynamically, we implement some complicated logic, but
it's buggy (results in a test failure in EE). Fixing it requires some
non-trivial effort, because a vtab may be changed after index creation
(when a space format is updated).

Let's drop this optimization altogether for now and use the same vtab
for both compressed and uncompressed indexes. We might return to this
issue in future, but first we need to run some benchmarks to check if
this optimization is worth the complexity. Possible ways how we could
resurrect this optimization:
 - Call get_raw from get directly (without function pointer), inline
   memtx_prepare_result_tuple, and move is_compressed flag to struct
   tuple for better cache locality.
 - Rebuild all tuples on space alter and use a different vtab for
   compressed indexes.

NO_DOC=bug fix
NO_TEST=enterprise
NO_CHANGELOG=unrelased