build: remove --enable-lto

This has outlived its usefulness, doesn't gel well with
newer compilers & `-flto` related options, i.e thin vs full, or `=auto`,
and having `-flto` as the only option means that sometimes this just
needs to be worked around, i.e in oss-fuzz:
https://github.com/google/oss-fuzz/blob/master/projects/bitcoin-core/build.sh.

While it was convenient when `-flto` was newer, support for `-flto` is now
in all compilers we use, and there's also no-longer any real need
for us to treat `-flto` different to any other optimization option.

Remove it, to remove build complexity, and so there's no need
to port a similar option to CMake.

Note that the LTO option remains in depends, because we still a way to
build packages that have LTO specific patches/options.

If we decide to merge this, I'll follow up downstream in oss-fuzz first,
to make sure we don't break the build.

depends/README.md

@@ -123,7 +123,7 @@ The following can be set when running make: `make FOO=bar`
 - `LOG`: Use file-based logging for individual packages. During a package build its log file
   resides in the `depends` directory, and the log file is printed out automatically in case
   of build error. After successful build log files are moved along with package archives
-- `LTO`: Use LTO when building packages.
+- `LTO`: Enable options needed for LTO. Does not add `-flto` related options to *FLAGS.
 - `NO_HARDEN=1`: Don't use hardening options when building packages
 
 If some packages are not built, for example `make NO_WALLET=1`, the appropriate