tdb2: remove tdb_get()

tdb2: remove tdb_get()

We have four internal helpers for reading data from the database:
1) tdb_read_convert() - read (and convert) into a buffer.
2) tdb_read_off() - read (and convert) and offset.
3) tdb_access_read() - malloc or direct access to the database.
4) tdb_get() - copy into a buffer or direct access to the database.

The last one doesn't really buy us anything, so remove it (except for
tdb_read_off/tdb_write_off, see next patch).

Before:
Adding 1000000 records:  6480 ns (59900296 bytes)
Finding 1000000 records:  2839 ns (59900296 bytes)
Missing 1000000 records:  2485 ns (59900296 bytes)
Traversing 1000000 records:  2598 ns (59900296 bytes)
Deleting 1000000 records:  5342 ns (59900296 bytes)
Re-adding 1000000 records:  5613 ns (59900296 bytes)
Appending 1000000 records:  12194 ns (93594224 bytes)
Churning 1000000 records:  14549 ns (93594224 bytes)

After:
Adding 1000000 records:  6497 ns (59900296 bytes)
Finding 1000000 records:  2854 ns (59900296 bytes)
Missing 1000000 records:  2563 ns (59900296 bytes)
Traversing 1000000 records:  2735 ns (59900296 bytes)
Deleting 1000000 records:  11357 ns (59900296 bytes)
Re-adding 1000000 records:  8145 ns (59900296 bytes)
Appending 1000000 records:  10939 ns (93594224 bytes)
Churning 1000000 records:  18479 ns (93594224 bytes)