[dbnode] Remove unused getMockReader

[linasm]

What this PR does / why we need it:
Noticed some more leftovers, cleaning up.

Special notes for your reviewer:

Does this PR introduce a user-facing and/or backwards incompatible change?:
NONE

Does this PR require updating code package or user-facing documentation?:
NONE

[codecov]

Codecov Report

Merging #3351 (561bdf4) into master (49f8c14) will decrease coverage by 0.0%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #3351     +/-   ##
=========================================
- Coverage    72.3%    72.3%   -0.1%     
=========================================
  Files        1098     1098             
  Lines      102107   102107             
=========================================
- Hits        73903    73889     -14     
- Misses      23103    23113     +10     
- Partials     5101     5105      +4     

Flag	Coverage Δ
aggregator	76.8% <ø> (-0.1%)	⬇️
cluster	84.9% <ø> (-0.1%)	⬇️
collector	84.3% <ø> (ø)
dbnode	78.9% <ø> (-0.1%)	⬇️
m3em	74.4% <ø> (ø)
m3ninx	73.6% <ø> (+<0.1%)	⬆️
metrics	19.8% <ø> (ø)
msg	74.4% <ø> (-0.2%)	⬇️
query	66.9% <ø> (ø)
x	80.5% <ø> (+0.1%)	⬆️

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