patch 9.0.2002: Vim9: need cleanup of class related interface code

patch 9.0.2002: Vim9: need cleanup of class related interface code

Problem:  Vim9: need cleanup of class related interface code
Solution: Remove the unused class variable and class method related code
          for interfaces.

Remove unused class variable and class method related code for
interfaces.

Refactor the code.

Optimize the object/class member double lookup in compile_lhs().

Change unused global functions to static functions.

closes: #13302

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Yegappan Lakshmanan <yegappan@yahoo.com>