remove process cancelling goroutine

[joshrwolf]

the goroutine is redundant, and blocks the actual return of the function when the ctx is done.

Error: -19T14:46:26.742Z [ERROR] provider.terraform-provider-oci_v0.0.17: failed to kill process: @caller=github.com/chainguard-dev/terraform-provider-oci/internal/provider/exec_test_data_source.go:194 error="os: process already finished" tf_data_source_type=oci_exec_test tf_provider_addr=registry.terraform.io/chainguard-dev/oci tf_req_id=c0da7202-2dff-f56f-aebc-de6d70d42686 @module=oci tf_rpc=ReadDataSource timestamp=2024-12-19T14:46:26.741Z

by removing it, we remove the duplicate cancellation handling (the timeout is already plumbed through the context), and the done signal is already handled by exec.CommandContext.

additionally, the process.Kill() is already handled by exec.CommandContext: https://cs.opensource.google/go/go/+/master:src/os/exec/exec.go;l=485