We do not need to pre-create user preferences, especially that creating
them too early creates one preferences for each person created in person
module and does not allow Person_createUserPreference to create
preference based on person's assignments are they are not created yet.

We do not need to clear cache when deleting user logins anymore. This
problem was fixed differently.

Adjust tests that was relying on cache being cleared when persons ar
deleted:

testERP5Web.TestERP5Web.test_15_Check_LastModified_Header was never
isolated from test_14.

test_14 was filling Base_getWebDocumentDrivenModificationDate cache and
this cache got clear when the persons where being deleted during
tearDown. When removing this interaction clearing cache, we revealed
this weakness. Choosen solution was to clear cache before checking
response headers, to make sure we don't get something filled by a
previous test.