Remove always-true if statement in encode_partition_topdown()

[redzic]

must_split is always false in this branch, because if it was true, the previous branch would have been taken.

[codecov-commenter]

Codecov Report

Merging #2980 (934f269) into master (97efc96) will decrease coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2980      +/-   ##
==========================================
- Coverage   86.58%   86.49%   -0.09%     
==========================================
  Files          87       87              
  Lines       33886    33879       -7     
==========================================
- Hits        29340    29304      -36     
- Misses       4546     4575      +29     

Impacted Files	Coverage Δ
src/encoder.rs	87.31% <100.00%> (-0.04%)	⬇️
src/context/frame_header.rs	66.06% <0.00%> (-2.27%)	⬇️
src/asm/x86/lrf.rs	91.59% <0.00%> (-1.90%)	⬇️
src/header.rs	68.93% <0.00%> (-1.65%)	⬇️
src/context/transform_unit.rs	89.78% <0.00%> (-0.72%)	⬇️
src/rdo.rs	85.74% <0.00%> (-0.05%)	⬇️
src/cpu_features/x86.rs	39.58% <0.00%> (ø)
v_frame/src/plane.rs	88.90% <0.00%> (+0.11%)	⬆️
src/context/mod.rs	92.63% <0.00%> (+1.05%)	⬆️

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