[CLN] Remove legacy backend and unused code (#1037)

# Description
Please include a summary of the changes.

Relates to <issue>

# Checklist
- [ ] My code uses type hinting for function and method arguments and return values.
- [ ] I have created tests which cover my code.
- [ ] The test code either 1. demonstrates at least one valuable use case (e.g. integration tests) 
or 2. verifies that outputs are as expected for given inputs (e.g. unit tests).
- [ ] New tests pass locally with my changes.

Author: Leguark

gempy/API/compute_API.py

@@ -56,9 +56,6 @@ def compute_model(gempy_model: GeoModel, engine_config: Optional[GemPyEngineConf
                 data_descriptor=gempy_model.input_data_descriptor,
                 geophysics_input=gempy_model.geophysics_input,
             )
-
-        case AvailableBackends.aesara | AvailableBackends.legacy:
-            gempy_model.legacy_model = _legacy_compute_model(gempy_model)
         case _:
             raise ValueError(f'Backend {engine_config} not supported')
 
@@ -189,9 +186,3 @@ def _check_convergence_criterion(conditional_number: float, condition_number_old
     return geo_model.solutions
 
 
-def _legacy_compute_model(gempy_model: GeoModel) -> 'gempy_legacy.Project':
-    gpl = require_gempy_legacy()
-    legacy_model: gpl.Project = gempy3_to_gempy2(gempy_model)
-    gpl.set_interpolator(legacy_model)
-    gpl.compute_model(legacy_model)
-    return legacy_model