fix: #260 remove dead receive_files() and mkdir_r()

receive_files() in src/server/server.c was a non-static, unprototyped,
zero-caller duplicate of receiver_process()/receiver_receive_files() in
src/server/receiver.c. Delete it along with the includes it uniquely pulled
(chunk.h, metadata.h, sys/stat.h, duplicate quoted unistd.h); remaining code
still uses file.h/config.h/protocol.h (via multiprocessing.h) directly.

mkdir_r() in src/shared/utils.c had zero callers in src/ and tests/; remove
the function and its declaration in utils.h, plus the unused libgen.h include.

src/shared/utils.h

@@ -5,7 +5,6 @@
 #include <stddef.h>
 #include <stdbool.h>
 
-bool mkdir_r(const char* path);
 char* str_dup(const char* string);
 char* output_escape(const char* string, bool eight_bit_output);
 char* path_cat(const char* path1, const char* path2);