Remove timetravel extension.

The extension depended on old types which are about to be removed. As
the code additionally was pretty crufty and didn't provide much in the
way of functionality, removing the extension seems to be the best way
forward.  It's fairly trivial to write functionality in plpgsql that
more than covers what timetravel did.

Author: Andres Freund
Discussion:
    https://postgr.es/m/20171213080506.cwjkpcz3bkk6yz2u@alap3.anarazel.de
    https://postgr.es/m/25615.1513115237@sss.pgh.pa.us

Author: anarazel

contrib/spi/Makefile

@@ -1,14 +1,13 @@
 # contrib/spi/Makefile
 
-MODULES = autoinc insert_username moddatetime refint timetravel
+MODULES = autoinc insert_username moddatetime refint
 
-EXTENSION = autoinc insert_username moddatetime refint timetravel
+EXTENSION = autoinc insert_username moddatetime refint
 
 DATA = autoinc--1.0.sql autoinc--unpackaged--1.0.sql \
        insert_username--1.0.sql insert_username--unpackaged--1.0.sql \
        moddatetime--1.0.sql moddatetime--unpackaged--1.0.sql \
-       refint--1.0.sql refint--unpackaged--1.0.sql \
-       timetravel--1.0.sql timetravel--unpackaged--1.0.sql
+       refint--1.0.sql refint--unpackaged--1.0.sql
 PGFILEDESC = "spi - examples of using SPI and triggers"
 
 DOCS = $(addsuffix .example, $(MODULES))