Replace coursecat class

[Info 3bits]

Adaptable running in moodle 3.6 shows:

Class coursecat is now alias to autoloaded class core_course_category, course_in_list is an alias to core_course_list_element. Class coursecat_sortable_records is deprecated without replacement. Do not include coursecatlib.php

As explained also in https://moodle.org/mod/forum/discuss.php?d=376752 coursecat class should be replaced by core_course_category

It is a blocker for moodle 3.6 compatibility

[Info 3bits]

fix #947 Remove coursecat class

→ <<cset 81982468822a>>

[Info 3bits]

fix #947 Add compatibility with moodle 3.5 and lower versions

→ <<cset 237c4ef6f72d>>

[Info 3bits]

fix #947 Add compatibility with moodle 3.5 and lower versions

→ <<cset 7f1307807fa8>>