Drop the ecdsa_preprocessing_data destructor (UB + heap leak)

[kozyilmaz]

It cleansed sizeof(struct) from k.data (== this), zeroing the vector/map control blocks before their destructors ran -- UB, leaking the buffers. The scalars already self-wipe and the containers are public, so it's unneeded.

This has been brought up by @nkavian, I kept the proving tests in a separate branch not to pollute your codebase. The test that proves the leak/UB lives in this branch kozyilmaz@5cb0aac

The fix is tested on the system below:

$ lsb_release -a
No LSB modules are available.
Distributor ID:	Ubuntu
Description:	Ubuntu 26.04 LTS
Release:	26.04
Codename:	resolute

$ uname -a
Linux utgard 7.0.0-22-generic #22-Ubuntu SMP PREEMPT_DYNAMIC Mon May 25 15:37:49 UTC 2026 aarch64 GNU/Linux

This is the test and output in the other branch (kozyilmaz@5cb0aac):

**Configure (one-time):**
cmake -S . -B build
**Build — just the leak test (fast):**
cmake --build build --target cosigner_preprocessing_data_leak_test -j$(nproc)
**Run — the leak test directly (clearest output):**
./build/test/cosigner/cosigner_preprocessing_data_leak_test

$ ./build/test/cosigner/cosigner_preprocessing_data_leak_test

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cosigner_preprocessing_data_leak_test is a Catch v2.13.8 host application.
Run with -? for options

-------------------------------------------------------------------------------
ecdsa_preprocessing_data destructor leaks its std::vector/std::map members
-------------------------------------------------------------------------------
/home/loki/devel/mpc-lib/test/cosigner/ecdsa_preprocessing_data_leak_test.cpp:53
...............................................................................

/home/loki/devel/mpc-lib/test/cosigner/ecdsa_preprocessing_data_leak_test.cpp:69: FAILED:
  REQUIRE( live == 0 )
with expansion:
  5 == 0
with message:
  heap allocations still live after destruction: 5 (a correct destructor frees
  them all, leaving 0)

===============================================================================
test cases: 1 | 1 failed
assertions: 1 | 1 failed

After this one line fix the test is cleared:

$ ./build/test/cosigner/cosigner_preprocessing_data_leak_test
===============================================================================
All tests passed (1 assertion in 1 test case)