Clean up CI and update OS check in elapsed module

- Removed redundant deploy workflow file from .github/workflows/deploy.yml 🗑️
- Updated POSIX OS define to ATHR_OS_UNIX in athr_elapsed.c 🔄

The deployment workflow was redundant and not used in the current CI process,
so it was removed to simplify maintenance. The POSIX OS check was updated
for clarity and consistency with the rest of the codebase.

athr_elapsed.c

@@ -4,7 +4,7 @@
 #error "C11 or a more recent version is required"
 #endif
 
-#if defined(POSIX_OS)
+#if ATHR_OS == ATHR_OS_UNIX
 #if !defined(_POSIX_C_SOURCE) || _POSIX_C_SOURCE < 200809L
 #undef _POSIX_C_SOURCE
 #define _POSIX_C_SOURCE 200809L