Revert "Add ocamlfind 1.5.1"

Reverts ocaml/opam-repository#2289

Suspicious failure on https://travis-ci.org/ocaml/opam-repository/jobs/28454399, reverting for now on:

```
===== ERROR while installing ocamlfind.1.5.1 =====
# opam-version 1.1.1
# os           linux
# command      make install
# path         /home/travis/.opam/system/build/ocamlfind.1.5.1
# compiler     system (4.01.0)
# exit-code    2
# env-file     /home/travis/.opam/system/build/ocamlfind.1.5.1/ocamlfind-7038-a00279.env
# stdout-file  /home/travis/.opam/system/build/ocamlfind.1.5.1/ocamlfind-7038-a00279.out
# stderr-file  /home/travis/.opam/system/build/ocamlfind.1.5.1/ocamlfind-7038-a00279.err
### stdout ###
# ...[truncated]
# # the following "if" block is only needed for 4.00beta2
# if [ 1 -eq 0 -a -f "/usr/lib/ocaml/compiler-libs/topdirs.cmi" ]; then \
# 	    cd "/usr/lib/ocaml/compiler-libs/"; \
# 	    cp topdirs.cmi toploop.cmi "/home/travis/.opam/system/lib/findlib/"; \
# 	fi
# make[1]: Leaving directory `/home/travis/.opam/system/build/ocamlfind.1.5.1/src/findlib'
# make[1]: Entering directory `/home/travis/.opam/system/build/ocamlfind.1.5.1/src/bytes'
# ocamlbuild -classic-display -no-links bytes.cmi bytes.cma
# cd _build/ && ocamlfind install bytes ../META bytes.cmi bytes.cma -optional bytes.cmx bytes.cmxa bytes.a bytes.cmxs
# make[1]: Leaving directory `/home/travis/.opam/system/build/ocamlfind.1.5.1/src/bytes'
### stderr ###
# Makefile:150: depend: No such file or directory
# File "topfind.ml", line 54, characters 10-19:
# Warning 26: unused variable stdlibdir.
# File "topfind.ml", line 116, characters 11-12:
# Warning 26: unused variable d.
# ocamlfind: Package bytes is already installed
#  - (file /home/travis/.opam/system/lib/bytes/META already exists)
# make[1]: *** [install] Error 2
# make: *** [install] Error 2
```