fix: fix warning about yyget_leng returning wrong type when using LTO

[szhorvat]

This attempt to fix the following errors found by CRAN:

https://www.stats.ox.ac.uk/pub/bdr/LTO/igraph.out

src/core/io/pajek-lexer.h:500:8: warning: type of ‘igraph_pajek_yyget_leng’ does not match original declaration [-Wlto-type-mismatch]
src/core/io/pajek-lexer.c:2457:11: note: return value type mismatch
src/core/io/pajek-lexer.c:2457:11: note: type ‘yy_size_t’ should match type ‘int’
src/core/io/pajek-lexer.c:2457:11: note: ‘igraph_pajek_yyget_leng’ was previously declared here
src/core/io/pajek-lexer.c:2457:11: note: code may be misoptimized unless ‘-fno-strict-aliasing’ is used

I noticed that this problem does not occur when compiling the C core separately. In fact, it seems that yyget_leng should return int instead fo yy_size_t.

This PR removes the string replacement that was meant to maintain compatibility with the ancient flex version that comes with macOS. Since we now generate parser sources with the latest flex on CI, I think there is no need for this fix anymore.

An alternative would be to avoid doing the replacement for yyget_leng.

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Note: I used the latest flex 2.6.4. for this. Not sure why it generated slightly different line numbers.