refactor: cleanup deadcode EnumDefinitionImpl

Module declaring `EnumDefinitionImpl` in
`linkml_runtime/utils/permissiblevalueimply.py` does not appear to be
used anywhere.

Additionally `EnumDefinitionImpl` already has a declaration in
`linkml_runtime/utils/enumerations.py`, which is being used in many
places in the code.

Signed-off-by: Silvano Cirujano Cuesta <silvano.cirujano-cuesta@siemens.com>

Author: Silvanoc