crimson: drop store from ECBackend to not shadow PGBackend::store

crimson: drop store from ECBackend to not shadow PGBackend::store

This problem was leading to crashes like this one:

```
../src/crimson/os/futurized_store.h:113:41: runtime error: member access within misaligned address 0xbebebebebebebebe for type 'struct Shard', which requires 8 byte alignment
0xbebebebebebebebe: note: pointer points here
<memory cannot be printed>
--Type <RET> for more, q to quit, c to continue without paging--

Thread 1 "crimson-osd" received signal SIGSEGV, Segmentation fault.
0x000055558e7a1dca in crimson::os::FuturizedStore::Shard::do_transaction (txn=..., ch=..., this=0xbebebebebebebebe) at ../src/crimson/os/futurized_store.h:113
113	      return do_transaction_no_callbacks(
(gdb) bt
```

Signed-off-by: Radosław Zarzyński <rzarzyns@redhat.com>