drop redundant type parameter

[cscherrer]

This PR deletes one token. Currently main contains this line:

Base.promote_typejoin(::Type{Either{<:Any, R}}, ::Type{Either{<:Any, R}}) where {L, R} = Either{<:Any, R}

The type parameter L is never bound, which can cause performance problems. This PR fixes that.

[codecov-commenter]

Codecov Report

Merging #3 (41f5a7c) into main (f3d74c6) will increase coverage by 0.05%.
The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##             main       #3      +/-   ##
==========================================
+ Coverage   84.77%   84.82%   +0.05%     
==========================================
  Files           8        9       +1     
  Lines         289      290       +1     
==========================================
+ Hits          245      246       +1     
  Misses         44       44              

Impacted Files	Coverage Δ
src/promote_type.jl	83.33% <100.00%> (ø)

... and 1 file with indirect coverage changes