Mk/Uses/cargo.mk: Drop stray backslashes from DEV_ERROR/WARNING messages

These appear as literal backslashes in the output rather than escaping
anything.

PR: 281600
Reviewed by: mikael (rust)
Approved by: mikael (rust)
MFH: 2026Q3

(cherry picked from commit f6977b775285d4bebb4711f17101fb12c636e1f9)