LDAP update following the st2 core integration from Enterprise

[arm4b]

The LDAP auth was integrated into the st2 core: StackStorm/st2#5082
Update LDAP documentation to remove any mentions of EWC/Enterprise and just sync it up.

• https://docs.stackstorm.com/authentication.html#ldap-enterprise-edition
• https://github.com/StackStorm/st2docs/blob/master/docs/source/authentication.rst

cc @hnanchahal