Remove unnecessary convert(Vector, wv) calls

[nalimilan]

AbstractWeights <: AbstractVector for a long time now. convert has to allocate a vector when UnitWeights is passed.

See #543.

[codecov]

Codecov Report

Merging #548 into master will decrease coverage by 0.09%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           master    #548     +/-   ##
========================================
- Coverage   90.39%   90.3%   -0.1%     
========================================
  Files          21      21             
  Lines        2104    2093     -11     
========================================
- Hits         1902    1890     -12     
- Misses        202     203      +1

Impacted Files	Coverage Δ
src/hist.jl	90.95% <ø> (-0.05%)	⬇️
src/cov.jl	95.91% <ø> (ø)	⬆️
src/sampling.jl	91.69% <100%> (-0.03%)	⬇️
src/moments.jl	99.3% <100%> (-0.03%)	⬇️
src/counts.jl	90.07% <100%> (-0.21%)	⬇️
src/weights.jl	86.05% <0%> (-0.49%)	⬇️

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