Removes assertions in IonReaderSystemTextX.stringValue().

[tgregg]

Issue #, if available:
Closes #702

Description of changes:
The assertion on line 609 was unnecessary; the assertion on line 611 was both wrong (should have been >=) and unnecessary because the next line throws an error that is more helpful than the assertion.

Before this change, the added test failed with AssertionError, which was wrong.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (dd2fdbf) 67.25% compared to head (df790b7) 67.26%.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #703      +/-   ##
============================================
+ Coverage     67.25%   67.26%   +0.01%     
- Complexity     5486     5488       +2     
============================================
  Files           159      159              
  Lines         23027    23025       -2     
  Branches       4128     4126       -2     
============================================
+ Hits          15486    15487       +1     
  Misses         6258     6258              
+ Partials       1283     1280       -3     

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[popematt]

Should we try to eliminate other asserts as well? (Not in this PR.)

[tgregg]

Should we try to eliminate other asserts as well? (Not in this PR.)

Generally, yes. There are a lot of usages. The ones I spot checked at least look like they're being used properly and won't surface in error conditions that can be triggered by users.