Revert "Use -flattenProvide: within -followedBy:"

This reverts commit f3116198a9d56b141897aabf581cdc907714d5b4.
Remove -flattenProvide:

Signal providers will often be used in circumstances that are more
appropriate for -concat or -switchToLatest, so this unnecessarily
obfuscates the -provide: call that users will need to understand.
Rename -promise: to -signalWithValue: