Remove unused `#include <stdarg.h>` in include/rocksdb/c.h (#11302)

Summary:
This include is unused in the header. In one build environment of ours, stdarg.h is actually not present, and this include prevents us from building rocksdb dependencies.

We're currently monkey-patching this line out in our build script (still WIP), which of course is not good. https://github.com/raipay/rust-rocksdb/commit/ec2852caa3074a3309881acf26284a60672e0b1b

Note that removing this include might break builds in unexpected ways that include rocksdb/c.h and then use `va_start`, `va_end`, etc. However, if you're using these functions, you really should include stdarg.h yourself, so I don't think this should prevent this PR.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11302

Reviewed By: ajkr

Differential Revision: D44139819

Pulled By: cbi42

fbshipit-source-id: 10c40b0b0260b23ccb7dc84e55a993c7dfbdc4cf

include/rocksdb/c.h

@@ -62,7 +62,6 @@
 extern "C" {
 #endif
 
-#include <stdarg.h>
 #include <stdbool.h>
 #include <stddef.h>
 #include <stdint.h>