fix(rust): remove redundant common shim declarations

The common C shims include headers that already declare every Rust-exported
symbol. Remove duplicate declarations so strict C builds do not report
redundant declarations or macro-expanded aliases.

The shims remain intentionally declaration-only; Rust still owns the exported
implementation.

Test Plan:
- compile debug.c with -Wall -Wextra -Werror
- compile error_private.c with -Wall -Wextra -Werror
- compile zstd_common.c with -Wall -Wextra -Werror

Refs: rust/README.md

lib/common/debug.c

@@ -1,4 +1,3 @@
 #include "debug.h"
 
 /* Implementation moved to Rust (rust/src/debug.rs) */
-extern int g_debuglevel;

lib/common/error_private.c

@@ -1,4 +1,3 @@
 #include "error_private.h"
 
-// Implementation moved to Rust
+/* Implementation moved to Rust (rust/src/errors.rs). */
-extern const char* ERR_getErrorString(ERR_enum code);

lib/common/zstd_common.c

@@ -2,9 +2,3 @@
 #include "zstd_internal.h"
 
 /* Implementation moved to Rust (rust/src/zstd_common.rs) */
-extern unsigned ZSTD_versionNumber(void);
-extern const char* ZSTD_versionString(void);
-extern unsigned ZSTD_isError(size_t code);
-extern const char* ZSTD_getErrorName(size_t code);
-extern ZSTD_ErrorCode ZSTD_getErrorCode(size_t code);
-extern const char* ZSTD_getErrorString(ZSTD_ErrorCode code);