Remove creation of plfs tmp directories in plfs.spec.

Since we are no longer providing plfsrc.example and putting it in as
/etc/plfsrc, there is no default plfs configuration. Therefore, it is
not necessary for the plfs rpm to do anything with the old plfs
directories.
Merge pull request #247 from dshrader/master

spec file updated to not create and remove default PLFS mount points and backends.