tests: bsim: bluetooth: mesh: Remove bsim tests with TinyCrypt

As TinyCrypt gets deprecated, we need to remove all bsim tests that use
it. This commit switches all Bluetooth mesh bsim tests to PSA as a
default crypto backend.

This includes:
- removing `CONFIG_BT_MESH_USES_TINYCRYPT` as PSA is default crypto
  backend for mesh. `CONFIG_BT_MESH_USES_MBEDTLS_PSA` is not required to
  be enabled;
- merging `overlay_psa.conf` into `prj.conf` as this configuration
  becomes default for all mesh bsim tests;
- merging `overlay_ss.conf` into `overlay_pst.conf` as secure storage is
  required for running persistent storage bsim tests with PSA;

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>