[#5081] Revert "Fix #4349 - Parsing with ignoreimport=true was always loading site.pp"

The fix for #4349 caused --parse-only not to detect syntax errors when
--ignore-import was used by adding a return statement that bypassed the
initial import:

commit 760e418d254a8d2198d2c6eb466d783a5930ef47
def perform_initial_import
+ return if Puppet.settings[:ignoreimport]

The problem that #4349 fixed was more generally fixed in commit
99c1019e1d3402ec8e476dc859d5aaef82ec4f69 for ticket #4798 so the return
statement is no longer needed, so reverting the commit for #4349 does
not reintroduce the problem of an import loop error when running puppet
doc.

Paired-with: Jesse Wolfe