refactor: Remove unnecessary code from proto/varlogpb/log_entry.go

[ijsong]

What this PR does

• replaced InvalidLogEntry with an empty LogEntry across the codebase
• removed proto/varlogpb/log_entry.go as it contained unused code
• updated related files to reflect these changes

[codecov]

Codecov Report

Attention: Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.

Project coverage is 67.30%. Comparing base (4499f90) to head (382277e).

Files with missing lines	Patch %	Lines
pkg/varlog/subscribe.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@                           Coverage Diff                            @@
##           tests_for_syncposition_and_syncrange     #927      +/-   ##
========================================================================
+ Coverage                                 67.20%   67.30%   +0.09%     
========================================================================
  Files                                       179      178       -1     
  Lines                                     21607    21599       -8     
========================================================================
+ Hits                                      14522    14538      +16     
+ Misses                                     6318     6305      -13     
+ Partials                                    767      756      -11     

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