curl: remove tool_writeenv.[ch]

... and USE_ENVIRONMENT and --environment. It was once added for RISC OS
support and its platform specific behavior has been annoying ever
since. Added in commit c3c8bbd3b2, mostly unchanged since
then. Most probably not actually used for years.

Closes #1463

docs/cmdline-opts/Makefile.inc

@@ -7,7 +7,7 @@ DPAGES = abstract-unix-socket.d anyauth.d append.d basic.d cacert.d capath.d cer
   data-raw.d data-urlencode.d delegation.d digest.d disable.d           \
   disable-eprt.d disable-epsv.d dns-interface.d dns-ipv4-addr.d         \
   dns-ipv6-addr.d dns-servers.d dump-header.d egd-file.d engine.d       \
-  environment.d expect100-timeout.d fail.d fail-early.d false-start.d   \
+  expect100-timeout.d fail.d fail-early.d false-start.d                 \
   form.d form-string.d ftp-account.d ftp-alternative-to-user.d          \
   ftp-create-dirs.d ftp-method.d ftp-pasv.d ftp-port.d ftp-pret.d       \
   ftp-skip-pasv-ip.d ftp-ssl-ccc.d ftp-ssl-ccc-mode.d ftp-ssl-control.d \

docs/cmdline-opts/environment.d

@@ -1,7 +0,0 @@
-Long: environment
-Help: Write results to environment variables
-Requires: RISC OS
----
-Sets a range of environment variables, using the names the --write-out option
-supports, to allow easier extraction of useful information after having run
-curl.