Remove numerous things deprecated during the 0.14 release cycle

Basically everything that has a direct replacement or no uses left.

Notable omissions:

- std.ArrayHashMap: Too much fallout, needs a separate cleanup.
- std.debug.runtime_safety: Too much fallout.
- std.heap.GeneralPurposeAllocator: Lots of references to it remain, not
  a simple find and replace as "debug allocator" is not equivalent to
  "general purpose allocator".
- std.io.Reader: Is being reworked at the moment.
- std.unicode.utf8Decode(): No replacement, needs a new API first.
- Manifest backwards compat options: Removal would break test data used
  by TestFetchBuilder.
- panic handler needs to be a namespace: Many tests still rely on it
  being a function, needs a separate cleanup.

test/cases/compile_errors/callconv_from_global_variable.zig

@@ -1,4 +1,4 @@
-var cc: @import("std").builtin.CallingConvention = .C;
+var cc: @import("std").builtin.CallingConvention = .c;
 export fn foo() callconv(cc) void {}
 
 // error

test/cases/compile_errors/implicit_cast_of_error_set_not_a_subset.zig

@@ -12,5 +12,5 @@ fn foo(set1: Set1) void {
 // backend=stage2
 // target=native
 //
-// :7:21: error: expected type 'error{C,A}', found 'error{A,B}'
+// :7:21: error: expected type 'error{A,C}', found 'error{A,B}'
 // :7:21: note: 'error.B' not a member of destination error set

test/cases/compile_errors/reify_type.Fn_with_is_generic_true.zig

@@ -1,6 +1,6 @@
 const Foo = @Type(.{
     .@"fn" = .{
-        .calling_convention = .Unspecified,
+        .calling_convention = .auto,
         .is_generic = true,
         .is_var_args = false,
         .return_type = u0,

test/cases/compile_errors/reify_type.Fn_with_is_var_args_true_and_non-C_callconv.zig

@@ -1,6 +1,6 @@
 const Foo = @Type(.{
     .@"fn" = .{
-        .calling_convention = .Unspecified,
+        .calling_convention = .auto,
         .is_generic = false,
         .is_var_args = true,
         .return_type = u0,

test/cases/compile_errors/reify_type.Fn_with_return_type_null.zig

@@ -1,6 +1,6 @@
 const Foo = @Type(.{
     .@"fn" = .{
-        .calling_convention = .Unspecified,
+        .calling_convention = .auto,
         .is_generic = false,
         .is_var_args = false,
         .return_type = null,