[INTERNAL] sap.m.ComboBox: do not clear the filter in .onBeforeOpen()

- the filter should be cleared only when needed and not when the
.open() method is called

Change-Id: I50a2efafe70b5a9142567a4148ddf93117ffa401
[FIX] sap.m: Remove dependencies to non-existent modules (e.g. enums)

Some UI5 entities like enums, simple types or interfaces, are no modules
on their own but embedded in some other module (most of the time this is
'library.js'). Dependencies to such non-modules should be avoided in
sap.ui.define or jQuery.sap.require calls as they can't be fulfilled
without hacks.

Change-Id: Id6e55c9e526fbf35a206d1ac6e226f18bc1969d2