Remove now useless 'canonicalize_backslashes: True' fields

Author: HugoGGuerrier

testsuite/tests/gnatcheck/lkql_rule_file_resolution/test.yaml

@@ -11,4 +11,3 @@ tests:
     project: prj/with_valid.gpr
   - label: With a project provided invalid rule file
     project: prj/with_invalid.gpr
-canonicalize_backslashes: True

testsuite/tests/gnatcheck_errors/already_existing_lkql_rule_file/test.yaml

@@ -5,4 +5,3 @@ rules:
 extra_args:
   - --emit-lkql-rule-file
 post_python: "cat('rules.lkql', sort=False)"
-canonicalize_backslashes: True

testsuite/tests/gnatcheck_errors/generic_inst_walking/test.yaml

@@ -3,4 +3,3 @@ project: prj.gpr
 extra_args: ['-l']
 rules:
   - +Rderiving_from_predefined_type
-canonicalize_backslashes: True

testsuite/tests/interpreter_errors/ambiguous_import/test.yaml

@@ -2,4 +2,3 @@ driver: 'interpreter'
 project: 'default_project/default.gpr'
 lkql_path:
   - others
-canonicalize_backslashes: True