get rid of unused exchange_alloc calloc method

this isn't actually calloc - it calls the malloc wrapper which no
longer zeroes

src/rt/rust_exchange_alloc.cpp

@@ -27,11 +27,6 @@ rust_exchange_alloc::malloc(size_t size) {
   return value;
 }
 
-void *
-rust_exchange_alloc::calloc(size_t size) {
-  return this->malloc(size);
-}
-
 void *
 rust_exchange_alloc::realloc(void *ptr, size_t size) {
   void *new_ptr = ::realloc(ptr, size);

src/rt/rust_exchange_alloc.h

@@ -17,7 +17,6 @@
 class rust_exchange_alloc {
  public:
     void *malloc(size_t size);
-    void *calloc(size_t size);
     void *realloc(void *mem, size_t size);
     void free(void *mem);
 };

src/rt/rust_kernel.cpp

@@ -79,11 +79,6 @@ rust_kernel::malloc(size_t size, const char *tag) {
     return exchange_alloc.malloc(size);
 }
 
-void *
-rust_kernel::calloc(size_t size, const char *tag) {
-    return exchange_alloc.calloc(size);
-}
-
 void *
 rust_kernel::realloc(void *mem, size_t size) {
     return exchange_alloc.realloc(mem, size);

src/rt/rust_kernel.h

@@ -133,7 +133,6 @@ public:
     void fatal(char const *fmt, ...);
 
     void *malloc(size_t size, const char *tag);
-    void *calloc(size_t size, const char *tag);
     void *realloc(void *mem, size_t size);
     void free(void *mem);
     rust_exchange_alloc *region() { return &exchange_alloc; }