-New Dell machine that shipped with two internal hard drives and arrived
in a RAID 1 configuration.

-I removed the RAID and some default recovery partitions and associated
drive letters with no issues. Well, *almost*.

On what is now a non-RAID, 500 GB "C" drive, there are 15 GB showing up
as "unallocated" (this used to be a Dell recovery partition of some
kind...but I manage my data backups separately).

Anyway to have those 15 GB returned to the OS/primary partition (without
formattting or adding a drive letter?

Under disk management, the only function being offered is new simple volume.

The 15 GB is negligible given the two large internal drives available...