Fix fstring errors in pyccel.ast (pyccel#1822)

Apply some more Pylint fixes. Namely:
- Fix fstring errors in pyccel.ast
- Fix several unused-import errors

Docstrings are also added to satisfy CI. Some deprecated code is
removed:
- `get_function_from_ast` which was unused

Author: EmilyBourne

CHANGELOG.md

@@ -45,6 +45,7 @@ All notable changes to this project will be documented in this file.
 -   \[INTERNALS\] Remove class `ast.datatypes.DataType` (replaced by `ast.datatypes.PrimitiveType` and `ast.datatypes.PyccelType`).
 -   \[INTERNALS\] Remove unused properties `prefix` and `alias` from `CustomDataType`.
 -   \[INTERNALS\] Remove `ast.basic.TypedAstNode._dtype`. The datatype can still be accessed as it is contained within the class type.
+-   \[INTERNALS\] Removed unused and undocumented function `get_function_from_ast`.
 
 ## \[1.11.2\] - 2024-03-05