collectd: modbus: Drop failing module that requires old lib

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  libmodbus v2.x is old and outdated, and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>