Revert "libpcp_pmda: change e_ext_t to expose pmdaInterface"

This reverts commit f856e2c17103540b87952e396f3a7de9cec9a66a.

Test failure is being observed in qa/628, this commit is the
current prime suspect (not sure why though, as yet):

$ ./check -q -l 628
628 27s ... - output mismatch (see 628.out.bad)
4c4
<     value 3
---
> No value(s) available!
16c16
<     value 4
---
> No value(s) available!

So, reverting temporarily and will reinstate after pcp-3.10.2
which is pending.
pmda papi: log errors that occur with no client context
docs: update changelog for pending release