Simplified thole_pol in bondfree.c.

Previously, the interaction called for four atoms involved
in Thole screening.  This is a bit inconvenient to deal with,
so now thole_pol() only acts on two atoms.  The topology should
list:

A1  A2
D1  D2
A1  D2
A2  D1

as the interactions for Thole screening, instead of all 4 in one
interaction.  Since thole_pol() is not used anywhere else in the
code, this change should be mostly harmless unless one is using
some very old topology that used the old format.

Change-Id: I2862df34f407260e1332abc6bcd8ae4ee93b7bc9