[CLN] Remove legacy backend and unused code

Eliminated "aesara" and "legacy" backends from the config and compute logic. Cleaned up redundant code, improving codebase maintainability and clarity by focusing on supported backends.

Author: Leguark

gempy_engine/config.py

@@ -8,10 +8,6 @@ class AvailableBackends(Flag):
     numpy = auto()
     PYTORCH = auto()
 
-    # Legacy
-    aesara = auto()
-    legacy = auto()
-
 
 # Define the paths for the .env files