Unify function declarations and definitions. (#896)

executable_semantics/ast/BUILD

@@ -32,10 +32,10 @@ cc_library(
     ],
     deps = [
         ":class_definition",
-        ":function_definition",
         ":member",
         ":pattern",
         ":source_location",
+        ":statement",
         "//common:ostream",
         "//executable_semantics/common:nonnull",
         "@llvm-project//llvm:Support",
@@ -67,18 +67,6 @@ cc_test(
     ],
 )
 
-cc_library(
-    name = "function_definition",
-    srcs = ["function_definition.cpp"],
-    hdrs = ["function_definition.h"],
-    deps = [
-        ":expression",
-        ":source_location",
-        ":statement",
-        "@llvm-project//llvm:Support",
-    ],
-)
-
 cc_library(
     name = "member",
     srcs = ["member.cpp"],