Drop TimeSpanMillisecondsConverterSharedTests after McpTask removal

PR #1579 (SEP-2663) replaced the SEP-1686 McpTask type with CreateTaskResult
and switched its ttl/pollInterval to bare `long?` properties, so the
TimeSpanMillisecondsConverter no longer has a second consumer. The shared
regression suite cherry-picked from PR #1623 references the now-removed
McpTask type and stops compiling.

The converter's clamp-instead-of-throw branches are still fully exercised by
CacheableResultTests (oversized, large-negative, +Inf, -Inf round-trips), so
no coverage is lost. Drop the file.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: halter73