Copter: remove unused hgt_variance_filt

[peterbarker]

this filter is populated but never checked when checking variances

../../ArduCopter/ekf_check.cpp:131:5: warning: Value stored to 'height_var' is never read [deadcode.DeadStores]
    height_var = hgt_variance_filt.apply(height_var, dt);
    ^            ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 warning generated.

[rmackay9]

Just for reference, here's the PR that added the filter