Revert "libmodbus: revert to 2.0.4 to unbreak collectd support"

This reverts commit 98d5c3a15acb46ffa557afbda30f2760ad66168c.

If collectd wants to stay on their own very outdated v4.10.x, that's
fine, but they should not drag other packages down around with them.
Support for modern versions of libmodbus has existed in collectd since
v5.0.0, released 2011-03-28, so the actively maintained version of
libmodbus in OpenWrt's modern package feed should remain at the modern
versions in use by third party applications.

This restores libmodbus to the existing version used
throughout AA and BB prior to this collectd "fix"

Signed-off-by: Karl Palsson <karlp@remake.is>
collectd: modbus: Drop failing module that requires old lib

collectd-mod-modbus in collectd versions prior to 5.0.0 require
libmodbus v2.0.x or earlier.  libmodbus v2.x is old and outdated, and
OpenWrt's packages feed has contained libmodbus v3.0.x for the entire
lifetime of Attitude Adjustment and most of Barrier Breaker.
Consequently, the collectd-mod-modbus plugin has never actually built
for this time, so rather than downgrading libmodbus, we simply remove
the collectd-mod-modbus plugin.

If collectd moves to version 5+, it gains support for modern versions of
libmodbus, and can be re-added.

Signed-off-by: Karl Palsson <karlp@remake.is>
Merge pull request #297 from remakeelectric/1407-collectd-modbus

Restore libmodbus and remove broken modbus collectd plugin