Bug 19166: Remove 'IF NOT EXISTS' in kohastructure

Bug 19166: Remove 'IF NOT EXISTS' in kohastructure

The table is removed just before, this clause is useless.

It also fixes a test:
t/db_dependent/Creators/Lib.t .. 1/645
  #   Failed test 'get_table_names return all tables matching'
  #   at t/db_dependent/Creators/Lib.t line 1258.
  #     Structures begin differing at:
  #          $got->[10] = 'aqinvoice_adjustments'
  #     $expected->[10] = 'aqinvoices'
  # Looks like you failed 1 test of 645.

Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>