Remove Microsoft/Cygwin import/export nonsense

Windows/Cygwin requires onerous special handling and the definition
of additional _imp__* symbols to import/export symbols between
dynamically linked binaries. Its support in AST used a lot of
macros and code obfuscation. In the features/common test for this,
AT&T called this the "Microsoft import/export nonsense".

They're right, it's nonsense. Somehow, Microsoft's POSIX layer,
SFU/Interix, always managed without it. No one has time to maintain
this (especially considering how incredibly sluggish Cygwin is).
And in fact, it had already fallen victim to bit rot; I confirmed
this in my early experiments with reintroducing dynamic library
support. No one has time to fix it, either.

So, my apologies to any Cygwin fans; ksh 93u+m will never support
dynamically loadable built-ins on Cygwin, even when I do manage to
reintroduce dynamic linking properly.

src/lib/libast/comp/execvpe.c

@@ -2,7 +2,7 @@
 *                                                                      *
 *               This software is part of the ast package               *
 *          Copyright (c) 1985-2011 AT&T Intellectual Property          *
-*          Copyright (c) 2020-2021 Contributors to ksh 93u+m           *
+*          Copyright (c) 2020-2022 Contributors to ksh 93u+m           *
 *                      and is licensed under the                       *
 *                 Eclipse Public License, Version 1.0                  *
 *                    by AT&T Intellectual Property                     *
@@ -31,17 +31,9 @@ NoN(execvpe)
 
 #else
 
-#if defined(__EXPORT__)
-__EXPORT__ int execvpe(const char*, char* const[], char* const[]);
-#endif
-
 #include <ast.h>
 #include <errno.h>
 
-#if defined(__EXPORT__)
-#define extern	__EXPORT__
-#endif
-
 extern int
 execvpe(const char* name, char* const argv[], char* const envv[])
 {