erasure-code: use new/delete to alloc/free coefficients array

erasure-code: use new/delete to alloc/free coefficients array

before this change, we allocate coefficients table with
`malloc()` in `ErasureCodeIsaDefault::prepare()`, but free them using
`delete`. this is identified by LeakSanitizer, and it reports

```
==3135332==ERROR: AddressSanitizer: alloc-dealloc-mismatch (malloc vs operator delete) on 0x60700002a870
    #0 0x5627c6ef721d in operator delete(void*) (/home/jenkins-build/build/workspace/ceph-pull-requests/build/bin/unittest_erasure_code_plugin_isa+0x1ca21d) (BuildId: 7922906370e5183d67f55211a868c0b0e22b4a2c)
    #1 0x7fbbe38e858f in ErasureCodeIsaTableCache::~ErasureCodeIsaTableCache() /home/jenkins-build/build/workspace/ceph-pull-requests/src/erasure-code/isa/ErasureCodeIsaTableCache.cc:65:13
    #2 0x7fbbe390be40 in ErasureCodePluginIsa::~ErasureCodePluginIsa() /home/jenkins-build/build/workspace/ceph-pull-requests/src/erasure-code/isa/ErasureCodePluginIsa.h:24:7
    #3 0x7fbbe390be68 in ErasureCodePluginIsa::~ErasureCodePluginIsa() /home/jenkins-build/build/workspace/ceph-pull-requests/src/erasure-code/isa/ErasureCodePluginIsa.h:24:7
    #4 0x5627c7063b52 in ceph::ErasureCodePluginRegistry::~ErasureCodePluginRegistry() /home/jenkins-build/build/workspace/ceph-pull-requests/src/erasure-code/ErasureCodePlugin.cc:49:5
    #5 0x7fbbeccb6494 in __run_exit_handlers stdlib/./stdlib/exit.c:113:8
    #6 0x7fbbeccb660f in exit stdlib/./stdlib/exit.c:143:3
    #7 0x7fbbecc9ad96 in __libc_start_call_main csu/../sysdeps/nptl/libc_start_call_main.h:74:3
    #8 0x7fbbecc9ae3f in __libc_start_main csu/../csu/libc-start.c:392:3
    #9 0x5627c6e38da4 in _start (/home/jenkins-build/build/workspace/ceph-pull-requests/build/bin/unittest_erasure_code_plugin_isa+0x10bda4) (BuildId: 7922906370e5183d67f55211a868c0b0e22b4a2c)
```

so, in this change, we use `new []` and `delete []` to allocate and free
them, to be more consistent, and to silence this warning.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>