Remove broken and undocumented tracing features

Remove broken and undocumented tracing features

Individual changes:
1. Drop -dtrace-scheme-coverage

As the initial commit dee82b1d0b of scm/coverage.scm notes, this
module requires support for memoize-symbol. This was indeed added
for Guile 1.9.0, but again removed in version 1.9.13. That means
there never was a stable version of Guile that worked with this.

When building / running with current Guile 1.8.8, the executable
crashes because the interpreter raise()s from within a critical
section and (at least my) libc kills the process:
 $ guile1.8 -c "(trap-enable 'memoize-symbol)"
throw from within critical section.
Aborted (core dumped)

(With later releases of Guile, there's at least an error message:
 $ guile2.0 -c "(trap-enable 'memoize-symbol)"
[...]
ERROR: In procedure memoize-variable-access!:
ERROR: Unbound variable: trap-enable

This is because the mentioned (unstable) release 1.9.13 completely
removed not only memoize-symbol, but the whole trap-* interface.)

2. Drop -dtrace-memory-frequency

The existing documentation is outdated:
 * The option is not bool, but takes an integer. Maybe the frequency,
   but in what unit?
 * The mentioned patch has been part of Guile since version 1.8.2.
   However I can't find the added variable since release 1.9.3, not
   unlikely that the module would not work with later versions.

Additionally there is no guide on how to use the produced files (.stacks
and .graph). If somebody has more information and wants this feature back,
please revert this commit and add appropriate documentation.

3. Delete scripts to measure coverage

build-coverage.sh is meant to configure LilyPond for gathering coverage
information, then runs a full 'make test' and finally calls coverage.py
to postprocess the data.

Apart from using the broken -dtrace-scheme-coverage, build-coverage.sh
only works when called from the repository's root directory (as opposed
to just creating a new build directory). Furthermore it randomly touches
midi2ly and its manpage midi2ly.1 which leads to errors at first.
Finally the called covarage.py doesn't seem to handle coverage data
emitted by the latest GCC, which makes it useless going forward.

4. Delete script build-profile.sh

It's not described in the documentation and seems to expect files
in input/mutopia which was removed back in 2009.

5. Update documentation about tracing options

[lemzwerg, 2020-01-19 06:33:32]

LGTM, thanks!