[Android] Remove unnecessary X509Chain memory leak test

[simonrozsival]

Follow-up to #128284
Follow-up to #128385

This PR removes an unnecessary long running unit test added in #128284

/cc @jkotas

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Pull request overview

This PR removes an Android-specific long-running X509Chain unit test from the System.Security.Cryptography test suite.

Changes:

• Deleted the Android-only stress test BuildChainRepeatedly_DoesNotExhaustGlobalReferences (including its helper local function) from ChainTests.cs.

Comments suppressed due to low confidence (2)

src/libraries/System.Security.Cryptography/tests/X509Certificates/ChainTests.cs:382

• Removing this method eliminates the only explicit Android regression coverage for the JNI global-ref leak scenario (no other references found in the tests). If the goal is to drop a 10-minute local stress test, consider replacing it with a CI-runnable check (or relocating it to an explicit stress/outerloop suite) so the original Android GREF overflow regression can’t silently reappear.

        [Fact]
        public static void BuildChainWithSystemTrustAndCustomTrustCertificates()
        {
            using (var testCert = new X509Certificate2(TestFiles.ChainPfxFile, TestData.ChainPfxPassword))

src/libraries/System.Security.Cryptography/tests/X509Certificates/ChainTests.cs:382

• After removing the Android stress test, the using System.Security.Cryptography.X509Certificates.Tests.Common; directive at the top of this file no longer appears to be referenced (no CertificateAuthority/PkiOptions/RevocationResponder usages remain). With TreatWarningsAsErrors enabled repo-wide, this will likely fail the build due to CS8019 unless the using is removed or the remaining usages are reintroduced.

        [Fact]
        public static void BuildChainWithSystemTrustAndCustomTrustCertificates()
        {
            using (var testCert = new X509Certificate2(TestFiles.ChainPfxFile, TestData.ChainPfxPassword))