Don't "upgrade" white imps to shadow/iron in Call Imp

Old frequencies at low (0-46) and high (92, near the cap of 100)
spell power:

          low    high
Crimson  20/36  10/36
White    10/36   5/36
Shadow    4/36  14/36
Iron      2/36   7/36

New frequencies at the same power levels:

          low    high
Crimson  20/36  10/36
White    10/36  10/36
Shadow    4/36 ~11/36 (actually 8/27)
Iron      2/36  ~5/36 (actually 4/27)

The breakpoint at 47 power (when shadow/iron imps start to become more
common) is still there.