MDL-40909 core_enrol: removed 'unenrol' add_to_log calls

Before each of these calls a call to unenrol_user() is made, which
triggers the event 'user_enrolment_deleted'. Nothing more is needed.
MDL-40909 core_enrol: removed 'enrol' add_to_log calls

Before each of these calls a call to enrol_user() is made, which
triggers the event 'user_enrolment_created'. Nothing more is needed.
MDL-40909 enrol_imsenterprise: replaced 'new' add_to_log call with an event