Remove explicit GOEXPERIMENT=systemcrypto

[qmuntal]

Microsoft build of Go maintainer here. SystemCrypto is enabled by default since Go 1.25, no need to set it manually (see docs). That experiment will most likely disappear in Go 1.27, so removing it now will safe you from a build error once upgrading to Go 1.27.

[jmclong]

@qmuntal, thanks for the PR. There's some internal tooling that isn't validly checking for FIPS-enablement unless the SYSTEMCRYPTO env var is set, so we had to add it back at least temporarily

@rnhan, have they resolved this issue yet?