Sample block cleanups (#657)

* Revert "[Bug 2533] New: Extreme space inefficiency importing silence from an AUP file"...

... It did not fix the symptoms.  Using a zero blob only gives temporary
in-memory space efficiency when adding a row to the database, not space savings
in the file.

This reverts commit d9047dfd25.

* Remove unused SampleBlockFactory::Get

* Eliminate some repetition and magic numbers

* Lower some common steps into GetSummary(); removed unused argument

* mSummary256Bytes and mSummary64kBytes not needed after initialization

* Satisfy sample and summary requests for silent blocks without using db

src/SampleBlock.h

@@ -116,9 +116,6 @@ public:
 
    virtual ~SampleBlockFactory();
 
-   // Returns a non-null pointer or else throws an exception
-   SampleBlockPtr Get(SampleBlockID sbid);
-
    // Returns a non-null pointer or else throws an exception
    SampleBlockPtr Create(samplePtr src,
       size_t numsamples,
@@ -139,10 +136,6 @@ public:
    virtual SampleBlockIDs GetActiveBlockIDs() = 0;
 
 protected:
-   // The override should throw more informative exceptions on error than the
-   // default InconsistencyException thrown by Create
-   virtual SampleBlockPtr DoGet(SampleBlockID sbid) = 0;
-
    // The override should throw more informative exceptions on error than the
    // default InconsistencyException thrown by Create
    virtual SampleBlockPtr DoCreate(samplePtr src,