Drop NumPy builds for Python 3.15 from CI

[leofang]

NumPy just had 3.15 nightlies. (Our 3.15 works actually started before NumPy was even ready, so we had to build NumPy in our CI and we can finally remove them now.)

[tirthpatel90]

Hi @leofang,

I would love to help with this!

I can remove the custom NumPy build steps for Python 3.15 from the CI workflow files so the pipeline uses the newly available NumPy 3.15 nightlies instead.

Could you please assign this issue to me? I'll submit a PR shortly.

[rparolin]

Hi @tirthpatel90. I'll assign this issue to you. Thank you for jumping in to help out! 👍