Merge bitcoin/bitcoin#29185: build: remove --enable-lto

2d1b1c7dae build: remove --enable-lto (fanquake)

Pull request description:

  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.

ACKs for top commit:
  TheCharlatan:
    ACK 2d1b1c7dae
  hebasto:
    ACK 2d1b1c7dae.

Tree-SHA512: 91812de7da35346f51850714a188fcffbac478bc8b348bf756c2555fcbde86ba622ac2fb77d294dea0378c741d3656f06121ef3a795aeed63fd170fc31bfa5af

depends/README.md

@@ -120,7 +120,7 @@ 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