fix: SQLAlchemy subquery and cache warnings

[hrideshmg]

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Should fix the following warnings:

var/www/sample-platform/mod_ci/controllers.py:220: SAWarning: TypeDecorator DeclEnumType('linux', 'windows') will not produce a cache key because the cache_ok attribute is not set to True. This can have significant performance implications including some performance degradations in comparison to prior SQLAlchemy versions. Set this attribute to True if this type object's state is safe to use in a cache key, or False to disable this warning. (Background on this error at: https://sqlalche.me/e/14/cprf)
maintenance_mode = MaintenanceMode.query.filter(MaintenanceMode.platform == platform).first()
/var/www/sample-platform/mod_ci/controllers.py:232: SAWarning: Coercing Subquery object into a select() for use in IN(); please pass a select() construct explicitly
Test.id.notin_(finished_tests), Test.id.notin_(running_tests), Test.platform == platform
/var/www/sample-platform/mod_ci/controllers.py:237: SAWarning: TypeDecorator DeclEnumType('preparation', 'testing', 'completed', 'canceled') will not produce a cache key because the cache_ok attribute is not set to True. This can have significant performance implications including some performance degradations in comparison to prior SQLAlchemy versions. Set this attribute to True if this type object's state is safe to use in a cache key, or False to disable this warning. (Background on this error at: https://sqlalche.me/e/14/cprf)
for test in pending_tests:
/var/www/sample-platform/mod_ci/controllers.py:237: SAWarning: TypeDecorator DeclEnumType('linux', 'windows') will not produce a cache key because the cache_ok attribute is not set to True. This can have significant performance implications including some performance degradations in comparison to prior SQLAlchemy versions. Set this attribute to True if this type object's state is safe to use in a cache key, or False to disable this warning. (Background on this error at: https://sqlalche.me/e/14/cprf)
for test in pending_tests:

Removed the conversion of the query object to a subquery as notin_ can directly use query objects.