MAJOR: http: Remove the HTTP legacy code

First of all, all legacy HTTP analyzers and all functions exclusively used by
them were removed. So the most of the functions in proto_http.{c,h} were
removed. Only functions to deal with the HTTP transaction have been kept. Then,
http_msg and hdr_idx modules were entirely removed. And finally the structure
http_msg was lightened of all its useless information about the legacy HTTP. The
structure hdr_ctx was also removed because unused now, just like unused states
in the enum h1_state. Note that the memory pool "hdr_idx" was removed and
"http_txn" is now smaller.
diff --git a/Makefile b/Makefile
index 54d345f..950cee5 100644
--- a/Makefile
+++ b/Makefile
@@ -774,8 +774,8 @@
        src/http_htx.o src/buffer.o src/hpack-tbl.o src/shctx.o src/sha1.o     \
        src/http.o src/hpack-dec.o src/action.o src/proto_udp.o src/http_acl.o \
        src/xxhash.o src/hpack-enc.o src/h2.o src/freq_ctr.o src/lru.o         \
-       src/protocol.o src/arg.o src/hpack-huff.o src/hdr_idx.o src/base64.o   \
-       src/hash.o src/mailers.o src/activity.o src/http_msg.o src/version.o   \
+       src/protocol.o src/arg.o src/hpack-huff.o src/base64.o                 \
+       src/hash.o src/mailers.o src/activity.o src/version.o                  \
        src/mworker.o src/mworker-prog.o src/debug.o src/wdt.o src/dict.o      \
        src/xprt_handshake.o