Remove alpha from nuget spec.
Remove alpha from nuget spec.

Alpha/beta labels will be appended by continuous integration (TeamCity) instead.

This simplifies a few pain points:
* Starting release branch, must remember to update nuspec from alpha to beta
* BEFORE finishing release branch, must remember to remove beta from nuspec
* AFTER finishing release branch, must add back alpha label to new nuspec back-merged from stable

Without alpha/beta semver labels checked in to code, we only have to remember bumping the version when starting a release/hotfix branch, which is standard procedure.

neutmute/UnitsNet,anjdreas/UnitsNet,anjdreas/UnitsNet,BoGrevyDynatest/UnitsNet,Tirael/UnitsNet,BrandonLWhite/UnitsNet,BrandonLWhite/UnitsNet,maherkassim/UnitsNet,maherkassim/UnitsNet